[PATCH, rs6000]Add support to enable vmsumudm behind vec_msum builtin.

[PATCH, rs6000]Add support to enable vmsumudm behind vec_msum builtin.

gcc/ChangeLog:

2020-07-08  Will Schmidt  <will_schmidt@vnet.ibm.com>

	* config/rs6000/altivec.h (vec_vmsumudm): New define.
	* config/rs6000/altivec.md (UNSPEC_VMSUMUDM): New unspec.
	  (altivec_vmsumudm): New define_insn.
	* config/rs6000/rs6000-builtin.def (altivec_vmsumudm): New BU_ALTIVEC_3
	  entry. (vmsumudm): New BU_ALTIVEC_OVERLOAD_3 entry.
	* config/rs6000/rs6000-call.c (altivec_overloaded_builtins): Add entries for
	  ALTIVEC_BUILTIN_VMSUMUDM variants of vec_msum.
	* doc/extend.texi: Add document for vmsumudm behind vmsum.

gcc/testsuite/ChangeLog:

2020-07-08  Will Schmidt  <will_schmidt@vnet.ibm.com>

	* gcc.target/powerpc/builtins-msum-runnable.c: New test.
	* gcc.target/powerpc/vsx-builtin-msum.c: New test.