Fix __atomic to not implement atomic loads with CAS.

gcc/
	* builtins.c (fold_builtin_atomic_always_lock_free): Make "lock-free"
	conditional on existance of a fast atomic load.
	* optabs-query.c (can_atomic_load_p): New function.
	* optabs-query.h (can_atomic_load_p): Declare it.
	* optabs.c (expand_atomic_exchange): Always delegate to libatomic if
	no fast atomic load is available for the particular size of access.
	(expand_atomic_compare_and_swap): Likewise.
	(expand_atomic_load): Likewise.
	(expand_atomic_store): Likewise.
	(expand_atomic_fetch_op): Likewise.
	* testsuite/lib/target-supports.exp
	(check_effective_target_sync_int_128): Remove x86 because it provides
	no fast atomic load.
	(check_effective_target_sync_int_128_runtime): Likewise.

libatomic/
	* acinclude.m4: Add #define FAST_ATOMIC_LDST_*.
	* auto-config.h.in: Regenerate.
	* config/x86/host-config.h (FAST_ATOMIC_LDST_16): Define to 0.
	(atomic_compare_exchange_n): New.
	* glfree.c (EXACT, LARGER): Change condition and add comments.

From-SVN: r245098