Enable store fusion on Power10.

Enable store fusion on Power10.

gcc/ChangeLog:

	* config/rs6000/rs6000-cpus.def (ISA_3_1_MASKS_SERVER): Add
	OPTION_MASK_P10_FUSION_2STORE.
	(POWERPC_MASKS): Likewise.
	* config/rs6000/rs6000.c (rs6000_option_override_internal): Enable
	store fusion for Power10.
	(is_fusable_store): New.
	(power10_sched_reorder): Likewise.
	(rs6000_sched_reorder): Do Power10 specific reordering.
	(rs6000_sched_reorder2): Likewise.
	* config/rs6000/rs6000.opt: Add new option.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/fusion-p10-stst.c: New test.
	* gcc.target/powerpc/fusion-p10-stst2.c: New test.