cmd/dist: enable -shared and external linking tests on s390x

Change-Id: Iedd01ef3a9d2831cb55c53b7a1984e7e932f4249
Reviewed-on: https://go-review.googlesource.com/20932Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -681,7 +681,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		return false
 	case "shared":
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le":
+		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-s390x":
 			return true
 		}
 		return false
@@ -725,7 +725,7 @@ func (t *tester) cgoTest(dt *distTest) error {
 	case "android-arm",
 		"dragonfly-386", "dragonfly-amd64",
 		"freebsd-386", "freebsd-amd64", "freebsd-arm",
-		"linux-386", "linux-amd64", "linux-arm",
+		"linux-386", "linux-amd64", "linux-arm", "linux-s390x",
 		"netbsd-386", "netbsd-amd64":
 
 		cmd := t.addCmd(dt, "misc/cgo/test", "go", "test", "-ldflags", "-linkmode=external")