Deletes the following s390x instructions:

 - ADDME
 - ADDZE
 - SUBME
 - SUBZE

They appear to be emulated PPC instructions left over from the
porting process and I don't think they will ever be useful.

Change-Id: I9b1ba78019dbd1218d0c8f8ea2903878802d1990
Reviewed-on: https://go-review.googlesource.com/30538
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>