So later than I wanted, here's the V2 of the stack clash mitigation work.
Probably the biggest change in this version was moving the protection
out of -fstack-check= and into its own option (-fstack-clash-protection)
This has been bootstrapped and regression tested on the same set of
targets {x86_64, powerpc, powerpc64le, aarch64, s390x}-linux-gnu. I've
also enabled -fstack-clash-protection and eyeballed test results
relative to the baselines to ensure nothing unexpected was failing.
Since this patch hits other targets that are not protected from
stack-clash a little harder, I also tested things like {alpha, mips,
ia64, hppa}-linux-gnu through building all-gcc.
As with the prior patch, comments, flames, questions are welcomed.
Jeff
