cmd/dist: disable plugin test on arm64

Mysterious error and no time or easy machine access to investigate
now, so disabling the -buildmode=plugin test on arm64. (The arm
version is working as expected.)

Updates #17138

Change-Id: I4cc56ddf47e7597213462e48d4934a765168bd07
Reviewed-on: https://go-review.googlesource.com/29363Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -746,8 +746,10 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		}
 		return false
 	case "plugin":
+		// linux-arm64 is missing because it causes the external linker
+		// to crash, see https://golang.org/issue/17138
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm", "linux-arm64":
+		case "linux-386", "linux-amd64", "linux-arm":
 			return true
 		}
 		return false