Allows the compiler to recognise s390x specific files and s390x
build tags.

Change-Id: I7c62ab7361cf708181b1d9cfbe9b1fcb01be31e0
Reviewed-on: https://go-review.googlesource.com/20872Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>