There is more zeroing than I would like right now -
temporaries used for the new map and channel runtime
calls need to be eliminated - but it will do for now.

This CL only has an effect if you are building with

        GOEXPERIMENT=precisestack ./all.bash

(or make.bash). It costs about 5% in the overall time
spent in all.bash. That number will come down before
we make it on by default, but this should be enough for
Keith to try using the precise maps for copying stacks.

amd64 only (and it's not really great generated code).

TBR=khr, iant
CC=golang-codereviews
https://golang.org/cl/56430043