cmd/internal/obj/arm: support new arm instructions

cmd/internal/obj/arm: support new arm instructions

There are two changes in this CL.

1. Add new forms of MOVH/MOVHS/MOVHU.
   MOVHS R0<<0(R1), R2   // ldrsh
   MOVH  R0<<0(R1), R2   // ldrsh
   MOVHU R0<<0(R1), R2   // ldrh
   MOVHS R2, R5<<0(R1)   // strh
   MOVH  R2, R5<<0(R1)   // strh
   MOVHU R2, R5<<0(R1)   // strh

2. Simpify "MVN $0xffffffaa, Rn" to "MOVW $0x55, Rn".
   It is originally assembled to two instructions.
   "MOVW offset(PC), R11"
   "MVN R11, Rn"

Change-Id: I8e863dcfb2bd8f21a04c5d627fa7beec0afe65fb
Reviewed-on: https://go-review.googlesource.com/53690
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>