libstdc++: Override BUFSIZ for Windows targets [PR 94268]

libstdc++: Override BUFSIZ for Windows targets [PR 94268]

This replaces uses of BUFSIZ with a new _GLIBCXX_BUFSIZ macro that can
be overridden in target-specific config headers.

That allows the mingw and mingw-w64 targets to override it, because
BUFSIZ is apparently defined to 512, resulting in poor performance. The
MSVCRT stdio apparently uses 4096, so we use that too.

libstdc++-v3/ChangeLog:

	PR libstdc++/94268
	* config/os/mingw32-w64/os_defines.h (_GLIBCXX_BUFSIZ):
	Define.
	* config/os/mingw32/os_defines.h (_GLIBCXX_BUFSIZ):
	Define.
	* include/bits/fstream.tcc: Use _GLIBCXX_BUFSIZ instead
	of BUFSIZ.
	* include/ext/stdio_filebuf.h: Likewise.
	* include/std/fstream (_GLIBCXX_BUFSIZ): Define.