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