cmd/cc: use a temporary bitmap when constructing pointer maps

cmd/cc: use a temporary bitmap when constructing pointer maps

This change makes the way cc constructs pointer maps closer to
what gc does and is being done in preparation for changes to
the internal content of the pointer map such as a change to
distinguish interface pointers from ordinary pointers.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12692043