testsuite: Simplify target test and dg-options for AMO tests

testsuite: Simplify target test and dg-options for AMO tests

Removed powerpc*-*-* from the target test as it is always true. Simplified
options by removing -mpower9-misc and -mvsx, which are enabled by default with
-mdejagnu-cpu=power9. The has_arch_pwr9 check is also true with
-mdejagnu-cpu=power9, so it has been removed.

2025-01-08 Jeevitha Palanisamy <jeevitha@linux.ibm.com>

gcc/testsuite/

	* gcc.target/powerpc/amo1.c: Removed powerpc*-*-* from the target and
	simplified dg-options.
	* gcc.target/powerpc/amo2.c: Simplified dg-options and added powerpc_vsx
	target check.