test/rs6000: Replace test targets p8 and p9+

test/rs6000: Replace test targets p8 and p9+

This patch is to clean existing rs6000 test targets p8 and p9+
with existing has_arch_pwr8 and has_arch_pwr9 targets combination
or only one of them.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr92398.p9+.c: Replace p9+ with has_arch_pwr9.
	* gcc.target/powerpc/pr92398.p9-.c: Replace p9+ with has_arch_pwr9,
	and replace p8 with has_arch_pwr8 && !has_arch_pwr9.
	* lib/target-supports.exp (check_effective_target_p8): Remove.
	(check_effective_target_p9+): Remove.