ld: make .rodata section read-only

Fixes the respective warning from elflint.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4607048

src/cmd/ld/data.c

@@ -781,7 +781,7 @@ dodata(void)
 	 */
 
 	/* read-only data */
-	sect = addsection(&segtext, ".rodata", 06);
+	sect = addsection(&segtext, ".rodata", 04);
 	sect->vaddr = 0;
 	datsize = 0;
 	s = datap;