libstdc++: Make _GLIBCXX_HOSTED respect -ffreestanding [PR103626]

This allows the library to switch to freestanding mode when compiling
with the -ffreestanding flag. This means you don't need a separate
libstdc++ build configured with --disable-hosted-libstdcxx in order to
compile for a freestanding environment.

The testsuite support files cannot be compiled for freestanding, so add
-fno-freestanding to override any -ffreestanding in the test flags.

libstdc++-v3/ChangeLog:

	PR libstdc++/103626
	* acinclude.m4 (GLIBCXX_ENABLE_HOSTED): Define _GLIBCXX_HOSTED
	to __STDC_HOSTED__ for non-freestanding installations.
	* configure: Regenerate.
	* include/Makefile.am (${host_builddir}/c++config.h): Adjust
	grep pattern.
	* include/Makefile.in: Regenerate.
	* testsuite/lib/libstdc++.exp (v3-build_support): Use
	-fno-freestanding.
	* testsuite/libstdc++-abi/abi.exp: Likewise.