Implements the following intrinsics on s390x:
 - AtomicAdd{32,64}
 - AtomicCompareAndSwap{32,64}
 - AtomicExchange{32,64}
 - AtomicLoad{32,64,Ptr}
 - AtomicStore{32,64,PtrNoWB}

I haven't added rules for And8 or Or8 yet.

Change-Id: I647af023a8e513718e90e98a60191e7af6167314
Reviewed-on: https://go-review.googlesource.com/31614
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>