Implements subset of MIPS32(r1) instruction set.

Change-Id: Iba017350f6c2763de05d4d1bc2f123e8eb76d0ff
Reviewed-on: https://go-review.googlesource.com/31475
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>