runtime: add s390x support (modified files only)

Change-Id: Ib79ad4a890994ad64edb1feb79bd242d26b5b08a
Reviewed-on: https://go-review.googlesource.com/20945
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/runtime/os2_linux_generic.go b/src/runtime/os2_linux_generic.go
index 01e6c8a..f1a2dd5 100644
--- a/src/runtime/os2_linux_generic.go
+++ b/src/runtime/os2_linux_generic.go
@@ -4,6 +4,7 @@
 
 // +build !mips64
 // +build !mips64le
+// +build !s390x
 // +build linux
 
 package runtime