cmd/gc: move large stack variables to heap

cmd/gc: move large stack variables to heap

Individual variables bigger than 10 MB are now
moved to the heap, as if they had escaped on
their own.

This avoids ridiculous stacks for programs that
do things like
        x := [1<<30]byte{}
        ... use x ...

If 10 MB is too small, we can raise the limit.

Fixes #6077.

R=ken2
CC=golang-dev
https://golang.org/cl/12650045