wasm2c warm.h: make panic a macro to propagate noreturn

I got the following warning when compiling zig-wasm2c:

C:\git\zig\stage1\wasm.h(55) : warning C4715: 'WasmMut_toC': not all control paths return a value

This is because the C compiler doesn't understand that the panic
function from `panic.h` is noreturn.  I addressed this making panic
a macro so the caller sees that abort is called and is able to see
that it's a "noreturn codepath".

Author: marler8997

stage1/panic.h

@@ -4,9 +4,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-static void panic(const char *reason) {
-    fprintf(stderr, "%s\n", reason);
-    abort();
-}
+// panic is a macro rather than a function so the caller can derive
+// that this is noreturn from the call to abort()
+#define panic(reason) do { fprintf(stderr, "%s\n", reason); abort(); } while (0)
 
 #endif /* PANIC_H */