rs6000: Add MMA __builtin_vsx_lxvp and __builtin_vsx_stxvp built-ins

rs6000: Add MMA __builtin_vsx_lxvp and __builtin_vsx_stxvp built-ins

Add support for the __builtin_vsx_lxvp and __builtin_vsx_stxvp built-ins
to keep in sync with LLVM which added these earlier.

2021-07-02  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-builtin.def (BU_MMA_PAIR_LD, BU_MMA_PAIR_ST):
	New macros.
	(__builtin_vsx_lxvp, __builtin_vsx_stxvp): New built-ins.
	* config/rs6000/rs6000-call.c (rs6000_gimple_fold_mma_builtin): Expand
	lxvp and stxvp built-ins.
	(mma_init_builtins): Handle lxvp and stxvp built-ins.
	(builtin_function_type): Likewise.
	* doc/extend.texi (__builtin_vsx_lxvp, __builtin_mma_stxvp): Document.

gcc/testsuite/
	* gcc.target/powerpc/mma-builtin-7.c: New test.
	* gcc.target/powerpc/mma-builtin-8.c: New test.

(cherry picked from commit 42a9e4e1381ba14d8ef21b331494945c2f51e6be)