BFX extracts given bits from the source register, sign extends them
to 32-bit, and writes to destination register. BFXU does the similar
operation with zero extention.

They were introduced in ARMv6T2.

Change-Id: I6822ebf663497a87a662d3645eddd7c611de2b1e
Reviewed-on: https://go-review.googlesource.com/56071
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>