runtime: avoid read overrun in heapdump
runtime: avoid read overrun in heapdump

Start the stack a few words below the actual top, so that
if something tries to read goexit's caller PC from the stack,
it won't fault on a bad memory address.
Today, heapdump does that.
Maybe tomorrow, traceback or something else will do that.
Make it not a bug.

TBR=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/136450043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go