SWPD/SWPW/SWPH/SWPB were introduced in ARMv8.1. They swap content
of register and memory atomically. And their difference is
SWPD: 64-bit double word data
SWPW: 32-bit word data (zero extended to 64-bit)
SWPH: 16-bit half word data (zero extended to 64-bit)
SWPB: 8-bit byte data (zero extended to 64-bit)

This CL implements them in the arm64 assembler.

Change-Id: I2d9fb2310674bd92693531210e187143e7eed602
Reviewed-on: https://go-review.googlesource.com/101516Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>