rs6000: Add xxgenpcvwm and xxgenpcvdm

rs6000: Add xxgenpcvwm and xxgenpcvdm

Add support for xxgenpcv[dw]m, along with individual and overloaded
built-in functions for access.

[gcc]

2020-05-11  Carl Love  <cel@us.ibm.com>

	* config/rs6000/altivec.h (vec_genpcvm): New #define.
	* config/rs6000/rs6000-builtin.def (XXGENPCVM_V16QI): New built-in
	instantiation.
	(XXGENPCVM_V8HI): Likewise.
	(XXGENPCVM_V4SI): Likewise.
	(XXGENPCVM_V2DI): Likewise.
	(XXGENPCVM): New overloaded built-in instantiation.
	* config/rs6000/rs6000-call.c (altivec_overloaded_builtins): Add
	entries for FUTURE_BUILTIN_VEC_XXGENPCVM.
	(altivec_expand_builtin): Add special handling for
	FUTURE_BUILTIN_VEC_XXGENPCVM.
	(builtin_function_type): Add handling for
	FUTURE_BUILTIN_XXGENPCVM_{V16QI,V8HI,V4SI,V2DI}.
	* config/rs6000/vsx.md (VSX_EXTRACT_I4): New mode iterator.
	(UNSPEC_XXGENPCV): New constant.
	(xxgenpcvm_<mode>_internal): New insn.
	(xxgenpcvm_<mode>): New expansion.
	* doc/extend.texi: Add documentation for vec_genpcvm built-ins.

[gcc/testsuite]

2020-05-11  Carl Love  <cel@us.ibm.com>

	* gcc.target/powerpc/xxgenpc-runnable.c: New.