cmd/internal/obj/arm64: add support for a series of load/store with register offset...

cmd/internal/obj/arm64: add support for a series of load/store with register offset instrucitons

The patch adds support for arm64 instructions LDRB, LDRH, LDRSB,
LDRSH, LDRSW, STR, STRB and STRH with register offset.

Test cases are also added.

Change-Id: I8d17fddd2963c0bc366e12b00bac49b93f3f0957
Reviewed-on: https://go-review.googlesource.com/91575
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>