openmp: Improve expand_omp_atomic_pipeline

openmp: Improve expand_omp_atomic_pipeline

When __atomic_* builtins were introduced, omp-expand.c (omp-low.c
at that point) has been adjusted in several spots so that it uses
the atomic builtins instead of sync builtins, but
expand_omp_atomic_pipeline has not because the __atomic_compare_exchange_*
APIs take address of the argument, so it kept using __sync_val_compare_swap_*.
That means it always uses seq_cst though.
This patch changes it to use the ATOMIC_COMPARE_EXCHANGE ifn which gimple-fold
folds __atomic_compare_exchange_* into - that ifn also passes expected
directly.

2021-09-03  Jakub Jelinek  <jakub@redhat.com>

	* omp-expand.c (expand_omp_atomic_pipeline): Use
	IFN_ATOMIC_COMPARE_EXCHANGE instead of
	BUILT_IN_SYNC_VAL_COMPARE_AND_SWAP_? so that memory order
	can be provided.