cmd/dist: actually disable testso and testsovar on ppc64

This is clearly what was intended all along. ./all.bash passes with this
change.

Change-Id: I16996da11cf1e4d2dc2a4434b7611a724691e8dc
Reviewed-on: https://go-review.googlesource.com/12068Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -734,7 +734,7 @@ func (t *tester) cgoTestSOSupported() bool {
 		// No exec facility on Android or iOS.
 		return false
 	}
-	if t.goos == "ppc64le" || t.goos == "ppc64" {
+	if t.goarch == "ppc64le" || t.goarch == "ppc64" {
 		// External linking not implemented on ppc64 (issue #8912).
 		return false
 	}