cmd/link/internal/ld: force external linking for mips64x with cgo

cgo internal linking is not supported yet (issue #14449).

Change-Id: Ic968916383d77b7f449db8f230c928a1e81939e0
Reviewed-on: https://go-review.googlesource.com/19807Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -571,7 +571,8 @@ func loadlib() {
 
 	// cmd/7l doesn't support cgo internal linking
 	// This is https://golang.org/issue/10373.
-	if iscgo && goarch == "arm64" {
+	// mips64x doesn't support cgo internal linking either (golang.org/issue/14449)
+	if iscgo && (goarch == "arm64" || goarch == "mips64" || goarch == "mips64le") {
 		Linkmode = LinkExternal
 	}