libstdc++: Test errno macros directly for all targets [PR 93151]

libstdc++: Test errno macros directly for all targets [PR 93151]

This applies the same changes to the djgpp and mingw versions of
error_constants.h as r11-6137 did for the generic version.

All of these constants are defined as macros by <errno.h> on these
targets, so we can just test the macro directly instead of checking for
it at configure time.

libstdc++-v3/ChangeLog:

	* config/os/djgpp/error_constants.h: Test POSIX errno macros
	directly, instead of corresponding _GLIBCXX_HAVE_EXXX macros.
	* config/os/mingw32-w64/error_constants.h: Likewise.
	* config/os/mingw32/error_constants.h: Likewise.