[libcxxabi] Define _LIBCXXABI_WEAK properly for mingw compilers

[mstorsjo]

Summary

Copy over the compiler detection structure from libcxx, and set _LIBCXXABI_WEAK like _LIBCPP_WEAK is set in libcxx.

This allows users to override operator new/delete, if using those operators from libcxxabi instead of from libcxx.

[ldionne]

Maybe we should actually add

#if defined(__clang__)
#  define _LIBCPPABI_COMPILER_CLANG
#  ifndef __apple_build_version__
#    define  _LIBCPPABI_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
#  endif
#elif defined(__GNUC__)
#  define  _LIBCPPABI_COMPILER_GCC
#elif defined(_MSC_VER)
#  define _LIBCPPABI_COMPILER_MSVC
#elif defined(__IBMCPP__)
#  define _LIBCPPABI_COMPILER_IBM
#endif

As a follow-up we should de-duplicate this (and a lot of other stuff) between libc++ and libc++abi. But if we copy-paste it in that way, at least it makes it clear that the logic is intended to be the same when we try to de-duplicate in the future. WDYT?

[mstorsjo]

In D89863#2344242, @ldionne wrote:

Maybe we should actually add

#if defined(__clang__)
#  define _LIBCPPABI_COMPILER_CLANG
#  ifndef __apple_build_version__
#    define  _LIBCPPABI_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
#  endif
#elif defined(__GNUC__)
#  define  _LIBCPPABI_COMPILER_GCC
#elif defined(_MSC_VER)
#  define _LIBCPPABI_COMPILER_MSVC
#elif defined(__IBMCPP__)
#  define _LIBCPPABI_COMPILER_IBM
#endif

As a follow-up we should de-duplicate this (and a lot of other stuff) between libc++ and libc++abi. But if we copy-paste it in that way, at least it makes it clear that the logic is intended to be the same when we try to de-duplicate in the future. WDYT?

Sure, that sounds good to me - but I'd use _LIBCXXABI as prefix as that's the spelling used within that lib, not _LIBCPPABI.

[mstorsjo]

Copied over the compiler detection logic from libcxx, to make them match more clearly.

libcxxabi/include/__cxxabi_config.h

#if defined(__arm__) && !defined(__USING_SJLJ_EXCEPTIONS__) &&                 \
     !defined(__ARM_DWARF_EH__)
 #define _LIBCXXABI_ARM_EHABI
 #endif
 
 #if !defined(__has_attribute)
 #define __has_attribute(_attribute_) 0
 #endif
 
+#if defined(__clang__)
+#  define _LIBCXXABI_COMPILER_CLANG
+#  ifndef __apple_build_version__
+#    define _LIBCXXABI_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
+#  endif
+#elif defined(__GNUC__)
+#  define _LIBCXXABI_COMPILER_GCC
+#elif defined(_MSC_VER)
+#  define _LIBCXXABI_COMPILER_MSVC
+#elif defined(__IBMCPP__)
+#  define _LIBCXXABI_COMPILER_IBM
+#endif
+
 #if defined(_WIN32)
  #if defined(_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS)
   #define _LIBCXXABI_HIDDEN
   #define _LIBCXXABI_DATA_VIS
   #define _LIBCXXABI_FUNC_VIS
   #define _LIBCXXABI_TYPE_VIS
  #elif defined(_LIBCXXABI_BUILDING_LIBRARY)
   #define _LIBCXXABI_HIDDEN

  #else
   #define _LIBCXXABI_HIDDEN
   #define _LIBCXXABI_DATA_VIS
   #define _LIBCXXABI_FUNC_VIS
   #define _LIBCXXABI_TYPE_VIS
  #endif
 #endif
 
-#if defined(_WIN32)
+#if defined(_LIBCXXABI_COMPILER_MSVC)
 #define _LIBCXXABI_WEAK
 #else
 #define _LIBCXXABI_WEAK __attribute__((__weak__))
 #endif
 
 #if defined(__clang__)
 #define _LIBCXXABI_COMPILER_CLANG
 #elif defined(__GNUC__)