backport: pa.md (memory_barrier): Revise to use ldcw barriers.

backport: pa.md (memory_barrier): Revise to use ldcw barriers.

	Backport from mainline
	2019-11-07  John David Anglin  <danglin@gcc.gnu.org>

	* config/pa/pa.md (memory_barrier): Revise to use ldcw barriers.
	Enhance comment.
	(memory_barrier_coherent, memory_barrier_64, memory_barrier_32): New
	insn patterns using ldcw instruction.
	(memory_barrier): Remove insn pattern using sync instruction.
	* config/pa/pa.opt (coherent-ldcw): New option.
	(ordered): New option.

From-SVN: r278001