cmd/gc: in the runtime package, don't promote any decls to the heap.

cmd/gc: in the runtime package, don't promote any decls to the heap.

In the runtime, we want to control where allocations happen.
In particular, we don't want the code implementing malloc to
itself trigger a malloc.  This change prevents the compiler
from inserting mallocs on our behalf (due to escaping declarations).

This check does not trigger on the current runtime code.

Note: Composite literals are still allowed.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/105280047