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>

src/runtime/runtime-gdb_test.go

diff --git a/src/runtime/runtime-gdb_test.go b/src/runtime/runtime-gdb_test.go
index 110d990..7cfcefc 100644
--- a/src/runtime/runtime-gdb_test.go
+++ b/src/runtime/runtime-gdb_test.go
@@ -104,7 +104,7 @@
 	// stack frames on RISC architectures.
 	canBackTrace := false
 	switch runtime.GOARCH {
-	case "amd64", "386", "ppc64", "ppc64le", "arm", "arm64", "mips64", "mips64le":
+	case "amd64", "386", "ppc64", "ppc64le", "arm", "arm64", "mips64", "mips64le", "s390x":
 		canBackTrace = true
 		args = append(args,
 			"-ex", "echo BEGIN goroutine 2 bt\n",