Load and store instructions are atomic on the s390x.

Change-Id: I0031ed2fba43f33863bca114d0fdec2e7d1ce807
Reviewed-on: https://go-review.googlesource.com/20938Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>