libstdc++: Fix freebsd/dragonfly build [PR116859]

As reported in the PR, the system headers libstdc++ changes result in
-Werror=expansion-to-defined errors on FreeBSD and supposedly on DragonFly
too.

The following patch fixes those by performing the preprocessor test right
away, rather than using defined in the macro definitions.

I think neither __ISO_C_VISIBLE nor __LONG_LONG_SUPPORTED should normally
change during compilation.

2024-09-26  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/116859
	* config/os/bsd/freebsd/os_defines.h
	(_GLIBCXX_USE_C99_LONG_LONG_DYNAMIC,
	_GLIBCXX_USE_C99_FLOAT_TRANSCENDENTALS_DYNAMIC): Avoid
	-Wexpansion-to-defined warnings.
	* config/os/bsd/dragonfly/os_defines.h
	(_GLIBCXX_USE_C99_LONG_LONG_DYNAMIC): Likewise.