cmd/link, runtime: add external linking support for linux/mips64x

Fixes #12560

Change-Id: Ic2004fc7b09f2dbbf83c41f8c6307757c0e1676d
Reviewed-on: https://go-review.googlesource.com/19803
Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -1298,6 +1298,8 @@ func hostlinkArchArgs() []string {
 		return []string{"-marm"}
 	case sys.ARM64:
 		// nothing needed
+	case '0':
+		return []string{"-mabi=64"}
 	}
 	return nil
 }