libstdc++: Combine duplicated tests for <atomic>

The types_std_c++0x.cc and types_std_c++20.cc tests are almost
identical, and the latter is redundant now that we can (and do) run all
tests for any -std option, and run with -std=gnu++20 by default.
Likewise for the _neg.cc tests.

Merge the C++20-specific parts into the c++0x tests and remove the c++20
ones.

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/headers/atomic/types_std_c++0x.cc: Check
	for C++20 std::memory_order enum. Use feature test macro for
	char8_t.
	* testsuite/29_atomics/headers/atomic/types_std_c++0x_neg.cc:
	Add check for atomic_char8_t.
	* testsuite/29_atomics/headers/atomic/types_std_c++20.cc:
	Removed.
	* testsuite/29_atomics/headers/atomic/types_std_c++20_neg.cc:
	Removed.
	* testsuite/29_atomics/headers/atomic/types_std_c++2a.cc:
	Removed.