libstdc++: Define <stdatomic.h> for C++23

This adds the C++23 <stdatomic.h> header, as proposed by P0943R6, for
compatibility with C code.

There are still some ABI differences between atomic_xxx in C and C++
std::atomic_xxx in C++, so this only provides source compatibility, not
binary compatibility.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Install new header.
	* include/Makefile.in: Regenerate.
	* include/c_compatibility/stdatomic.h: New file.
	* testsuite/29_atomics/headers/stdatomic.h/c_compat.cc: New test.