cmd/internal/obj/s390x: add atomic operation instructions

cmd/internal/obj/s390x: add atomic operation instructions

Adds the following s390x instructions from the interlocked access
facility:

 * LAA(G)  - load and add
 * LAAL(G) - load and add logical
 * LAN(G)  - load and and
 * LAX(G)  - load and exclusive or
 * LAO(G)  - load and or

These instructions can be used for atomic arithmetic/logical
operations. The atomic packages will be updated in future CLs.

Change-Id: Idc850ac6749b3e778fda3da66bcd864f6b1df375
Reviewed-on: https://go-review.googlesource.com/27871
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>