MSYS2: Export forwarders for __cxa_begin_catch, __cxa_end_catch, __cxa_rethrow and __gxx_personality_seh0

libobjc2 uses native C++ exceptions on MinGW.  The clang compiler will emit references to `__cxa_begin_catch`, `__cxa_end_catch`, `__cxa_rethrow` and `__gxx_personality_seh0` for Objective C code which uses Objective C exceptions.

These symbols are defined in the C++ runtime, not in libobjc2.  As a result, merely linking with libobjc2 is not sufficient.  Objective C code such as GNUstep must be compiled with the `LDFLAGS="-lgcc_s -lstdc++"` or `LDFLAGS="-lc++"`, depending on the environment.

This is tedious.  Additionally, specifying `-lc++` on the msys/clang64 environment causes linker errors:

```
 Linking library libgnustep-base ...
ld.lld: error: libc++.dll.a(libc++.dll): .idata$4 should not refer to special section 0
```

A [similar error has been observed for other libraries](msys2/MINGW-packages#18589)

A solution for this is to define forwarding exports for `__cxa_begin_catch`, `__cxa_end_catch`, `__cxa_rethrow` and `__gxx_personality_seh0`.  This is implemented by adding a `eh_forwards.def` file to the list of libobjc2 source files, which forwards the symbols to the actual C++ runtime.  On MSYS2, the libstdc++ and libc++ runtimes are supported, which covers all MinGW environments: https://www.msys2.org/docs/environments/.

Forwarding exports are discussed here:
- https://learn.microsoft.com/en-us/cpp/build/reference/exports?view=msvc-170
- https://devblogs.microsoft.com/oldnewthing/20060719-24/?p=30473
- https://devblogs.microsoft.com/oldnewthing/20121116-00/?p=6073

Author: qmfrederik

.github/workflows/main.yml

@@ -202,13 +202,12 @@ jobs:
         include:
           - msystem: ucrt64
             package-prefix: ucrt-x86_64
-            cmake-flags: LDFLAGS="-fuse-ld=lld -lstdc++ -lgcc_s"
+            cmake-flags: LDFLAGS="-fuse-ld=lld"
           - msystem: mingw64
             package-prefix: x86_64
-            cmake-flags: LDFLAGS="-fuse-ld=lld -lstdc++ -lgcc_s"
+            cmake-flags: LDFLAGS="-fuse-ld=lld"
           - msystem: clang64
             package-prefix: clang-x86_64
-            cmake-flags: LDFLAGS="-lc++"
             ctest-flags: -E UnexpectedException*
       # Don't abort runners if a single one fails
       fail-fast: false

CMakeLists.txt

@@ -28,6 +28,7 @@ endif ()
 
 INCLUDE (CheckCXXSourceCompiles)
 INCLUDE (FetchContent)
+INCLUDE (CheckCXXSymbolExists)
 
 set(libobjc_VERSION 4.6)
 
@@ -130,6 +131,29 @@ else ()
 	list(APPEND libobjc_C_SRCS eh_personality.c)
 endif ()
 
+# Export the __cxa_begin_catch, __cxa_end_catch and __gxx_personality_seh0 symbols from libobjc2
+# and forward them to the C runtime
+if (MINGW)
+	check_cxx_symbol_exists(_GLIBCXX_RELEASE "version" HAVE_LIBSTDCXX)
+	check_cxx_symbol_exists(_LIBCPP_VERSION "version" HAVE_LIBCXX)
+
+	if (HAVE_LIBSTDCXX)
+		find_library(CXX_RUNTIME "stdc++" REQUIRED)
+		set(CXX_RUNTIME_NAME "libstdc++-6")
+	elseif(HAVE_LIBCXX)
+		find_library(CXX_RUNTIME "c++" REQUIRED)
+		get_filename_component(CXX_RUNTIME_NAME ${CXX_RUNTIME} NAME_WE CACHE)
+	else ()
+		message(WARNING "Could not determine the C++ runtime.")
+	endif ()
+
+	if (CXX_RUNTIME)
+		message(STATUS "Adding forwarders for __cxa_begin_catch, __cxa_end_catch, __cxa_rethrow and __gxx_personality_seh0 to ${CXX_RUNTIME_NAME}")
+		configure_file(eh_forwards.def.in ${CMAKE_CURRENT_BINARY_DIR}/eh_forwards.def @ONLY)
+		list(APPEND libobjc_CXX_SRCS ${CMAKE_CURRENT_BINARY_DIR}/eh_forwards.def)
+	endif()
+endif ()
+
 find_package(tsl-robin-map)
 
 if (NOT tsl-robin-map_FOUND)

eh_forwards.def.in

@@ -0,0 +1,5 @@
+EXPORTS
+ __cxa_begin_catch = @CXX_RUNTIME_NAME@.__cxa_begin_catch
+ __cxa_end_catch = @CXX_RUNTIME_NAME@.__cxa_end_catch
+ __cxa_rethrow = @CXX_RUNTIME_NAME@.__cxa_rethrow
+ __gxx_personality_seh0 = @CXX_RUNTIME_NAME@.__gxx_personality_seh0

objcxx_eh.cc

@@ -463,6 +463,7 @@ PRIVATE void cxx_throw()
 	throw x;
 }
 
+#ifndef __MINGW32__
 /**
  * Personality function that wraps the C++ personality and inspects the C++
  * exception structure on the way past.  This should be used only for the
@@ -490,7 +491,6 @@ BEGIN_PERSONALITY_FUNCTION(test_eh_personality)
  * personality function, allowing us to inspect a C++ exception that is in a
  * known state.
  */
-#ifndef __MINGW32__
 extern "C" void test_cxx_eh_implementation()
 {
 	if (done_setup)

objcxx_eh.h

@@ -14,7 +14,13 @@ extern "C" {
 #undef CXA_ALLOCATE_EXCEPTION_SPECIFIER
 #define CXA_ALLOCATE_EXCEPTION_SPECIFIER
 #endif
-__attribute__((weak))
+
+#ifndef __MINGW32__
+#define OBJC_WEAK __attribute__((weak))
+#else
+#define OBJC_WEAK
+#endif
+OBJC_WEAK
 void *__cxa_allocate_exception(size_t thrown_size) CXA_ALLOCATE_EXCEPTION_SPECIFIER;
 
 /**
@@ -23,32 +29,33 @@ void *__cxa_allocate_exception(size_t thrown_size) CXA_ALLOCATE_EXCEPTION_SPECIF
  * _Unwind_Exception structure within this structure, and should be passed to
  * the C++ personality function.
  */
-__attribute__((weak))
+OBJC_WEAK
 struct _Unwind_Exception *objc_init_cxx_exception(id thrown_exception);
 /**
  * The GNU C++ exception personality function, provided by libsupc++ (GNU) or
  * libcxxrt (PathScale).
  */
-__attribute__((weak)) DECLARE_PERSONALITY_FUNCTION(__gxx_personality_v0);
+OBJC_WEAK
+DECLARE_PERSONALITY_FUNCTION(__gxx_personality_v0);
 /**
  * Frees an exception object allocated by __cxa_allocate_exception().  Part of
  * the Itanium C++ ABI.
  */
-__attribute__((weak))
+OBJC_WEAK
 void __cxa_free_exception(void *thrown_exception);
 /**
  * Tests whether a C++ exception contains an Objective-C object, and returns if
  * if it does.  The second argument is a pointer to a boolean value indicating
  * whether this is a valid object.
  */
-__attribute__((weak))
+OBJC_WEAK
 void *objc_object_for_cxx_exception(void *thrown_exception, int *isValid);
 
 /**
  * Prints the type info associated with an exception.  Used only when
  * debugging, not compiled in the normal build.
  */
-__attribute__((weak))
+OBJC_WEAK
 void print_type_info(void *thrown_exception);
 
 /**