cmd/dist: generate offsets for P structure
cmd/dist: generate offsets for P structure

So far no checked-in assembly needs these,
but it matches having them for M and G.
I needed these for some manual testing.

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/11595043

apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go