cmd/internal/obj/arm: support more ARMv5/ARMv6/ARMv7 instructions

cmd/internal/obj/arm: support more ARMv5/ARMv6/ARMv7 instructions

REV/REV16/REVSH were introduced in ARMv6, they offered more efficient
byte reverse operatons.

MMUL/MMULA/MMULS were introduced in ARMv6, they simplified
a serial of mul->shift->add/sub operations into a single instruction.

RBIT was introduced in ARMv7, it inversed a 32-bit word's bit order.

MULS was introduced in ARMv7, it corresponded to MULA.

MULBB/MULABB were introduced in ARMv5TE, they performed 16-bit
multiplication (and accumulation).

Change-Id: I6365b17b3c4eaf382a657c210bb0094b423b11b8
Reviewed-on: https://go-review.googlesource.com/35565
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>