rs6000: Add new __builtin_vsx_build_pair and __builtin_mma_build_acc built-ins

rs6000: Add new __builtin_vsx_build_pair and __builtin_mma_build_acc built-ins

The __builtin_vsx_assemble_pair and __builtin_mma_assemble_acc built-ins
currently assign their first source operand to the first VSX register
in a pair/quad, their second operand to the second register in a pair/quad, etc.
This is not endian friendly and forces the user to generate different calls
depending on endianness.  In agreement with the POWER LLVM team, we've
decided to lightly deprecate the assemble built-ins and replace them with
"build" built-ins that automatically handle endianness so the same built-in
call and be used for both little-endian and big-endian compiles.  We are not
removing the assemble built-ins, since there is code in the wild that use
them, but we are removing their documentation to encourage the use of the
new "build" variants.

gcc/
	* config/rs6000/rs6000-builtin.def (build_pair): New built-in.
	(build_acc): Likewise.
	* config/rs6000/rs6000-call.c (mma_expand_builtin): Swap assemble
	source operands in little-endian mode.
	(rs6000_gimple_fold_mma_builtin): Handle VSX_BUILTIN_BUILD_PAIR.
	(mma_init_builtins): Likewise.
	* config/rs6000/rs6000.c (rs6000_split_multireg_move): Handle endianness
	ordering for the MMA assemble and build source operands.
	* doc/extend.texi (__builtin_vsx_build_acc, __builtin_mma_build_pair):
	Document.
	(__builtin_mma_assemble_acc, __builtin_mma_assemble_pair): Remove
	documentation.

gcc/testsuite/
	* gcc.target/powerpc/mma-builtin-4.c (__builtin_vsx_build_pair): Add
	tests.  Update expected counts.
	* gcc.target/powerpc/mma-builtin-5.c (__builtin_mma_build_acc): Add
	tests.  Update expected counts.