Repeat previous fix on amd64 for s390x.
Sub-word right shifts should sign extend before shifting.

Update #23812

Change-Id: I2d770190c7d8a22310b0dbd9facb3fb05afa362a
Reviewed-on: https://go-review.googlesource.com/94028
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>