PR c/68908
	* c-typeck.c (build_atomic_assign): Improve commentary.  Add
	optimization to use __atomic_fetch_* built-in if possible.

	* gcc.dg/atomic/c11-atomic-exec-6.c: New test.
	* gcc.dg/atomic/c11-atomic-exec-7.c: New test.
	* gcc.dg/atomic/stdatomic-op-5.c: New test.

From-SVN: r232052