cmd/internal/obj/arm: support more ARMv6 instructions

cmd/internal/obj/arm: support more ARMv6 instructions

The following instructions were introduced in ARMv6, and the compiler
can do more optimization with them.

1. "MOVBS Rm@>i, Rd": rotates Rm 0/8/16/24 bits, does signed
byte extension to word, and writes the result to Rd.

2. "MOVHS Rm@>i, Rd": rotates Rm 0/8/16/24 bits, does signed
halfword extension to word, and writes the result to Rd.

3. "MOVBU Rm@>i, Rd": rotates Rm 0/8/16/24 bits, does unsigned
byte extension to word, and writes the result to Rd.

4. "MOVHU Rm@>i, Rd": rotates Rm 0/8/16/24 bits, does unsigned
half-word extension to word, and writes the result to Rd.

5. "XTAB Rm@>i, Rn, Rd": rotates Rm 0/8/16/24 bits, does signed
byte extension to word, adds Rn, and writes the result to Rd.

6. "XTAH Rm@>i, Rn, Rd": rotates Rm 0/8/16/24 bits, does signed
half-word extension to word, adds Rn, and writes the result to Rd.

7. "XTABU Rm@>i, Rn, Rd": rotates Rm 0/8/16/24 bits, does unsigned
byte extension to word, adds Rn, and writes the result to Rd.

8. "XTAHU Rm@>i, Rn, Rd": rotates Rm 0/8/16/24 bits, does unsigned
half-word extension to word, adds Rn, and writes the result to Rd.

Change-Id: I4306d7ebac93015d7e2e40d307f2c4271c03f466
Reviewed-on: https://go-review.googlesource.com/65790
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>