cmd/dist: disable plugin test on ppc64le and s390x

These are close to working, but untested and failing on
build.golang.org. So disable for now.

Change-Id: I330b8d1a91f0bf5139c894913868f01ec87e986d
Reviewed-on: https://go-review.googlesource.com/29359Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/test.go

@@ -729,7 +729,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		return false
 	case "plugin":
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64", "linux-ppc64le", "linux-s390x":
+		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64":
 			return true
 		}
 		return false