cmd/internal/obj/arm64: add LDPW/LDPSW/STPW to arm64 assembler

cmd/internal/obj/arm64: add LDPW/LDPSW/STPW to arm64 assembler

1. STPW stores the lower 32-bit words of a pair of registers to memory.
2. LDPW loads two 32-bit words from memory, zero extends them to 64-bit,
and then copies to a pair of registers.
3. LDPSW does the same as LDPW, except a sign extension.

This CL implements those 3 instructions and adds test cases.

Change-Id: Ied9834d8240240d23ce00e086b4ea456e1611f1a
Reviewed-on: https://go-review.googlesource.com/99956
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>