cmd/gc: do not generate dead value maps yet
cmd/gc: do not generate dead value maps yet

We are not clearing dead values in the garbage collector so it
is not worth the RSS cost to materialize the data and write it
out to the binary.

R=golang-dev, iant, cshapiro
CC=golang-dev
https://codereview.appspot.com/38650043

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