Add -Wtsan.

gcc/ChangeLog:

	PR sanitizer/97868
	* common.opt: Add new warning -Wtsan.
	* doc/invoke.texi: Likewise.
	* tsan.c (instrument_builtin_call): Warn users about unsupported
	std::atomic_thread_fence.

gcc/testsuite/ChangeLog:

	PR sanitizer/97868
	* gcc.dg/tsan/atomic-fence.c: New test.