runtime: reenable TestStackGrowth on 32-bit systems

If this needs to be turned back off, it should be done
just before the '// in finalizer' comment, not at the top
of the function.

GC is more precise now than it was (the only imprecise
stuff left is some global variables), so maybe the finalizer
test will work now on 32-bit systems.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/144030043