rs6000: add effective-target test ppc_mma_hw

rs6000: add effective-target test ppc_mma_hw

Add a test for dejagnu to determine if execution of MMA instructions is
supported in the test environment. Add an execution test to make sure
that __builtin_cpu_supports("mma") is true if we can execute MMA
instructions.

gcc/testsuite/

	* lib/target-supports.exp (check_ppc_mma_hw_available):
	New function.
	(is-effective-target): Add ppc_mma_hw.
	(is-effective-target-keyword): Add ppc_mma_hw.
	* gcc.target/powerpc/mma-supported.c: New file.
	* gcc.target/powerpc/mma-single-test.c: Require ppc_mma_hw.
	* gcc.target/powerpc/mma-double-test.c: Require ppc_mma_hw.