cmd/dist: make raceDetectorSupported an exact copy of cmd/internal/sys.RaceDetectorSupported

The comment states that cmd/internal/sys.RaceDetectorSupported is a copy,
so make the two identical. No functional difference, since ppce64le is
only supported on linux anyway.

Change-Id: Id3e4d445fb700b9b3bb53bf15ea05b8911b4f95e
Reviewed-on: https://go-review.googlesource.com/134595
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -1469,8 +1469,10 @@ func (t *tester) packageHasBenchmarks(pkg string) bool {
 // because cmd/dist has to be buildable by Go 1.4.
 func raceDetectorSupported(goos, goarch string) bool {
 	switch goos {
-	case "linux", "darwin", "freebsd", "netbsd", "windows":
+	case "linux":
 		return goarch == "amd64" || goarch == "ppc64le"
+	case "darwin", "freebsd", "netbsd", "windows":
+		return goarch == "amd64"
 	default:
 		return false
 	}