libstdc++: Add atomic_fetch_xor to <stdatomic.h>

libstdc++: Add atomic_fetch_xor to <stdatomic.h>

This function (and the explicit memory over version) are present in both
C++ <atomic> and C <stdatomic.h>, so should be in C++ <stdatomic.h> too.
There is a library issue incoming for this, but the resolution is
obvious.

libstdc++-v3/ChangeLog:

	* include/c_compatibility/stdatomic.h (atomic_fetch_xor): Add
	using-declaration.
	(atomic_fetch_xor_explicit): Likewise.
	* testsuite/29_atomics/headers/stdatomic.h/c_compat.cc: Check
	arithmetic and logical operations for atomic_int.