libstdc++: Fix unintended layout change to std::basic_filebuf [PR108331]

GCC 13 has a new implementation of gthr-win32.h which supports C++11
mutexes, threads etc. but this causes an unintended ABI break. The
__gthread_mutex_t type is always used in std::basic_filebuf even in
C++98, so independent of whether C++11 sync primitives work or not.
Because that type changed for the win32 thread model, we have a layout
change in std::basic_filebuf. The member is completely unused, it just
gets passed to the std::__basic_file constructor and ignored. So we
don't need that mutex to actually work, we just need its layout to not
change.

Introduce a new __gthr_win32_legacy_mutex_t struct in gthr-win32.h with
the old layout, and conditionally use that in std::basic_filebuf.

	PR libstdc++/108331

libgcc/ChangeLog:

	* config/i386/gthr-win32.h (__gthr_win32_legacy_mutex_t): New
	struct matching the previous __gthread_mutex_t struct.
	(__GTHREAD_LEGACY_MUTEX_T): Define.

libstdc++-v3/ChangeLog:

	* config/io/c_io_stdio.h (__c_lock): Define as a typedef for
	__GTHREAD_LEGACY_MUTEX_T if defined.