libstdc++: Fix tests that fail with -fno-char8_t

libstdc++: Fix tests that fail with -fno-char8_t

Adjust expected errors or skip tests as UNSUPPORTED if -fno-char8_t is
used in the test flags.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/integer_comparisons/equal_neg.cc: Use
	no-opts selector for errors that depend on -fchar8_t.
	* testsuite/20_util/integer_comparisons/greater_equal_neg.cc:
	Likewise.
	* testsuite/20_util/integer_comparisons/greater_neg.cc:
	Likewise.
	* testsuite/20_util/integer_comparisons/in_range_neg.cc:
	Likewise.
	* testsuite/20_util/integer_comparisons/less_equal_neg.cc:
	Likewise.
	* testsuite/20_util/integer_comparisons/less_neg.cc: Likewise.
	* testsuite/20_util/integer_comparisons/not_equal_neg.cc:
	Likewise.
	* testsuite/21_strings/basic_string/hash/hash_char8_t.cc: Skip
	if -fno-char8_t is used.
	* testsuite/21_strings/headers/cuchar/functions_std_cxx20.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_character/char/deleted.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_character/wchar_t/deleted.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/factory/u8path-depr.cc: Use
	char for u8 literal if char8_t is not available.
	* testsuite/27_io/headers/iosfwd/synopsis.cc: Check
	__cpp_char8_t.
	* testsuite/29_atomics/atomic_integral/wait_notify.cc: Likewise.
	* testsuite/29_atomics/headers/atomic/types_std_c++20_neg.cc:
	Remove check for _GLIBCXX_USE_CHAR8_T.