cmd/dist: allow gohostarch to be s390x

Should let the s390x builder progress a little further.

Change-Id: I5eab5f384b0b039f8e246ba69ecfb24de08625d2
Reviewed-on: https://go-review.googlesource.com/20965Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/util.go

@@ -452,6 +452,8 @@ func main() {
 			} else {
 				gohostarch = "mips64le"
 			}
+		case strings.Contains(out, "s390x"):
+			gohostarch = "s390x"
 		case gohostos == "darwin":
 			if strings.Contains(run("", CheckExit, "uname", "-v"), "RELEASE_ARM_") {
 				gohostarch = "arm"