Use plq/pstq for atomic_{load,store}<mode>

Use plq/pstq for atomic_{load,store}<mode>

For future architecture with prefix instructions, always use plq/pstq
rather than lq/stq for atomic load of quadword. Then we never have to
do the doubleword swap on little endian. Before this fix, -mno-pcrel
would generate lq with the doubleword swap (which was ok) and -mpcrel
would generate plq, also with the doubleword swap, which was wrong.

2020-04-20  Aaron Sawdey  <acsawdey@linux.ibm.com>

	PR target/94622
	* config/rs6000/sync.md (load_quadpti): Add attr "prefixed"
	if TARGET_PREFIXED.
	(store_quadpti): Ditto.
	(atomic_load<mode>): Do not swap doublewords if TARGET_PREFIXED as
	plq will be used and doesn't need it.
	(atomic_store<mode>): Ditto, for pstq.