Define LLVM_ABI and CLANG_ABI for __EMSCRIPTEN__ builds (#131578)

While building llvm (clang, lld) against emscripten we see this
[error](https://github.com/emscripten-forge/recipes/actions/runs/13803029307/job/38608794602#step:9:1715)

```
 │ │ In file included from $SRC_DIR/llvm/lib/Frontend/OpenACC/ACC.cpp:9:
 │ │ $SRC_DIR/build/include/llvm/Frontend/OpenACC/ACC.h.inc:192:1: error: unknown type name 'LLVM_ABI'
 │ │   192 | LLVM_ABI Directive getOpenACCDirectiveKind(llvm::StringRef Str);
 │ │       | ^
 │ │ $SRC_DIR/build/include/llvm/Frontend/OpenACC/ACC.h.inc:192:19: error: expected ';' after top level declarator
 │ │   192 | LLVM_ABI Directive getOpenACCDirectiveKind(llvm::StringRef Str);
 │ │       |                   ^
 ```

Now this was happening because we weren't defining LLVM_ABI correctly when building against emscripten. If you see [llvm/Support/Compiler.h](https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Support/Compiler.h#L206-L210), the condition only checked for the platform __WASM__ . Now Emscripten targets WebAssembly but doesn't imply the platform by default so the check isn't complete to define LLVM_ABI.

The successful build after using this patch can be seen [here](https://github.com/emscripten-forge/recipes/actions/runs/13805214092/job/38614585621)

(cherry picked from commit e57cd10)

Author: anutosh491

clang/include/clang/Support/Compiler.h

@@ -54,7 +54,7 @@
 #define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_EXPORT_TEMPLATE
-#elif defined(__MACH__) || defined(__WASM__)
+#elif defined(__MACH__) || defined(__WASM__) || defined(__EMSCRIPTEN__)
 #define CLANG_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define CLANG_TEMPLATE_ABI
 #define CLANG_EXPORT_TEMPLATE

llvm/include/llvm/Support/Compiler.h

@@ -203,7 +203,7 @@
 #define LLVM_TEMPLATE_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_EXPORT_TEMPLATE
 #define LLVM_ABI_EXPORT LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
-#elif defined(__MACH__) || defined(__WASM__)
+#elif defined(__MACH__) || defined(__WASM__) || defined(__EMSCRIPTEN__)
 #define LLVM_ABI LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #define LLVM_TEMPLATE_ABI
 #define LLVM_EXPORT_TEMPLATE