libsupc++: Make the destructor parameter to `__cxa_thread_atexit()` use the...

libsupc++: Make the destructor parameter to `__cxa_thread_atexit()` use the `__thiscall` calling convention for i686-w64-mingw32

The mingw-w64 implementations of `__cxa_thread_atexit()` and `__cxa_atexit()` have been
using `__thiscall` since two years ago. Using the default calling convention (which is
`__cdecl`) causes crashes as explained in PR83562.

Calling conventions have no effect on x86_64-w64-mingw32.

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83562
Reference: https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/mingw-w64-crt/crt/cxa_thread_atexit.c
Reference: https://sourceforge.net/p/mingw-w64/mingw-w64/ci/f3e0fbb40cbc9f8821db8bd8a0c4dae8ff671e9f/
Reference: https://github.com/msys2/MINGW-packages/issues/7071


Signed-off-by: Liu Hao <lh_mouse@126.com>

    2020-10-08  Liu Hao  <lh_mouse@126.com>

	libstdc++-v3:
	* libsupc++/cxxabi.h: (__cxa_atexit): mark with _GLIBCXX_CDTOR_CALLABI
	(__cxa_thread_atexit): ditto
	* libsupc++/atexit_thread.cc: (__cxa_atexit): mark with
	_GLIBCXX_CDTOR_CALLABI
	(__cxa_thread_atexit): ditto
	(elt): ditto