cmd/dist: enable race detector test on S390X

The support was added but the test was not enabled. Enable it.

Fixes #53981.

Change-Id: I81ea73ea4ebc1013c35cb70ae88b096e02497887
Reviewed-on: https://go-review.googlesource.com/c/go/+/418914
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/dist/test.go

@@ -1720,7 +1720,7 @@ func (t *tester) runPrecompiledStdTest(timeout time.Duration) error {
 func raceDetectorSupported(goos, goarch string) bool {
 	switch goos {
 	case "linux":
-		return goarch == "amd64" || goarch == "ppc64le" || goarch == "arm64"
+		return goarch == "amd64" || goarch == "ppc64le" || goarch == "arm64" || goarch == "s390x"
 	case "darwin":
 		return goarch == "amd64" || goarch == "arm64"
 	case "freebsd", "netbsd", "openbsd", "windows":