runtime: efence support for growable stacks

runtime: efence support for growable stacks
1. Fix the bug that shrinkstack returns memory to heap.
   This causes growslice to misbehave (it manually initialized
   blocks, and in efence mode shrinkstack's free leads to
   partially-initialized blocks coming out of growslice.
   Which in turn causes GC to crash while treating the garbage
   as Eface/Iface.
2. Enable efence for stack segments.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://golang.org/cl/74080043