libstdc++: Make some tests work on freestanding [PR103626]

libstdc++: Make some tests work on freestanding [PR103626]

PR libstdc++/103626 - _GLIBCXX_HOSTED should respect -ffreestanding

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/103626
	* testsuite/17_intro/headers/c++1998/stdc++.cc [!__STDC_HOSTED__]:
	Do not include C headers that aren't valid for freestanding.
	* testsuite/17_intro/tag_type_explicit_ctor.cc [!__STDC_HOSTED__]:
	Do not test tag types that aren't defined for freestanding.
	* testsuite/18_support/headers/cstdlib/functions_std.cc: Do not
	check for std::getenv and std::system for freestanding.
	* testsuite/17_intro/using_namespace_std_exp_neg.cc [!__STDC_HOSTED__]:
	Do not test hosted parts of the standard library.
	* testsuite/17_intro/using_namespace_std_tr1_neg.cc [!__STDC_HOSTED__]:
	Likewise.
	* testsuite/20_util/allocator_traits/members/rebind_alloc.cc [!__STDC_HOSTED__]:
	Likewise.
	* testsuite/20_util/allocator_traits/requirements/explicit_instantiation.cc [!HOSTED]:
	Likewise.
	* testsuite/20_util/headers/bitset/synopsis.cc [!__STDC_HOSTED__]: Likewise.
	* testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc [!__STDC_HOSTED__]:
	Likewise.
	* testsuite/20_util/pointer_traits/requirements/typedefs.cc [!__STDC_HOSTED__]:
	Likewise.
	* testsuite/20_util/tuple/cons/deduction.cc [!__STDC_HOSTED__]: Likewise.
	* testsuite/25_algorithms/move/93872.cc [!__STDC_HOSTED__]: Likewise.
	* testsuite/std/ranges/adaptors/100577.cc [!__STDC_HOSTED__]: Likewise.