ld: make .rodata section read-only
ld: make .rodata section read-only

Fixes the respective warning from elflint.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4607048

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go