This is a AMD64 version of CL19743.
Saves additional 1574 bytes in go binary.
This also speeds up bzip2 by 1-4%

Change-Id: I031ba423663c4e83fdefe44e5296f24143e303da
Reviewed-on: https://go-review.googlesource.com/19939
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>