gopack: omit time stamps
gopack: omit time stamps

This makes it possible to build a package twice
and get the same bytes both times.

R=r, dsymonds, r2
CC=golang-dev
http://codereview.appspot.com/4248077

dsriram/golang,apparentlymart/golang,ssrl/go,gnanderson/go,pombredanne/go-deleteme,ssrl/go,ssrl/go,gnanderson/go,apparentlymart/golang,ssrl/go,ssrl/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,ssrl/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go