cmd/internal/obj/arm: add BFC/BFI to arm's assembler

cmd/internal/obj/arm: add BFC/BFI to arm's assembler

BFC (Bit Field Clear) and BFI (Bit Field Insert) were
introduced in ARMv6T2, and the compiler can use them
to do further optimization.

Change-Id: I5a3fbcd2c2400c9bf4b939da6366c854c744c27f
Reviewed-on: https://go-review.googlesource.com/72891
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>