archive/zip: add File.DataOffset

Accessor to find where the bytes of a file start.

R=golang-dev, rsc, dsymonds, adg
CC=golang-dev
https://codereview.appspot.com/12784045
runtime: fix build on arm

Do not use ? :
I cannot say this enough.

TBR=dvyukov
CC=golang-dev
https://codereview.appspot.com/12903043