libstdc++: Combine two std::from_chars tests into one

libstdc++: Combine two std::from_chars tests into one

We don't need separate tests for the C++17 and C++20 cases, we can just
have one test that uses __cpp_char8_t to adjust whether it tests char8_t
or not. This means the C++20 one doesn't fail if -fno-char8_t is used.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/from_chars/1_neg.cc: Add char8_t cases,
	using a struct of that name if -fno-char8_t is active.
	* testsuite/20_util/from_chars/1_c++20_neg.cc: Removed.