cmd/link: fix external linker argument for mips64

I overlooked it when rebasing CL 19803.

Change-Id: Ife9d6bcc6a772715d137af903c64bafac0cdb216
Reviewed-on: https://go-review.googlesource.com/22797Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -1302,7 +1302,7 @@ func hostlinkArchArgs() []string {
 		return []string{"-marm"}
 	case sys.ARM64:
 		// nothing needed
-	case '0':
+	case sys.MIPS64:
 		return []string{"-mabi=64"}
 	}
 	return nil