This patch introduces the stack clash protection options

Changes since V2:

Adds two new params.  The first controls the size of the guard area.
This controls the threshold for when a function prologue requires
probes.  The second controls the probing interval -- ie, once probes are
needed, how often do we emit them.  These are really meant more for
developers to experiment with than users.  Regardless I did go ahead and
document them./PARAM

It also adds some sanity checking WRT combining stack clash protection
with -fstack-check.

Jeff