rs6000: Add support for vmsumcud and vec_msumc

2022-02-08  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-builtins.def (VMSUMCUD): New.
	* config/rs6000/rs6000-overload.def (VEC_MSUMC): New.
	* config/rs6000/vsx.md (UNSPEC_VMSUMCUD): New constant.
	(vmsumcud): New define_insn.

gcc/testsuite/
	* gcc.target/powerpc/vec-msumc.c: New test.