cmd/asm,cmd/internal/obj/ppc64: recognize ppc64 ISA 3.1 MMA registers

cmd/asm,cmd/internal/obj/ppc64: recognize ppc64 ISA 3.1 MMA registers

Allow the assembler frontend to match MMA register arguments added by
ISA 3.1. The prefix "A" (for accumulator) is chosen to identify them.

Updates #44549

Change-Id: I363e7d1103aee19d7966829d2079c3d876621efc
Reviewed-on: https://go-review.googlesource.com/c/go/+/419534
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>