Improve atomic store implementation on hppa-linux.

2021-12-27  John David Anglin  <danglin@gcc.gnu.org>

gcc/ChangeLog:

	* config/pa/pa-protos.h: Delete
	pa_maybe_emit_compare_and_swap_exchange_loop() declaration.
	* config/pa/pa.c (pa_expand_compare_and_swap_loop): Delete.
	(pa_maybe_emit_compare_and_swap_exchange_loop): Delete.
	* config/pa/pa.md (atomic_storeq): Use __sync_lock_test_and_set
	instead of pa_maybe_emit_compare_and_swap_exchange_loop.
	(atomic_storehi, atomic_storesi, atomic_storedi): Likewise.