rs6000: Make the has_arch target selectors actually work

rs6000: Make the has_arch target selectors actually work

2022-04-26  Segher Boessenkoool  <segher@kernel.crashing.org>

gcc/testsuite/
	PR target/105349
	* lib/target-supports.exp (check_effective_target_has_arch_pwr5): Use
	the specified dg-options.
	(check_effective_target_has_arch_pwr6): Ditto.
	(check_effective_target_has_arch_pwr7): Ditto.
	(check_effective_target_has_arch_pwr8): Ditto.
	(check_effective_target_has_arch_pwr9): Ditto.
	(check_effective_target_has_arch_pwr10): Ditto.
	(check_effective_target_has_arch_ppc64): Ditto.