Protection against stack-based memory corruption errors using SafeStack:...

Protection against stack-based memory corruption errors using SafeStack: compiler-rt runtime support library

This patch adds runtime support for the Safe Stack protection to compiler-rt
(see http://reviews.llvm.org/D6094 for the detailed description of the
Safe Stack).

This patch is our implementation of the safe stack on top of compiler-rt. The
patch adds basic runtime support for the safe stack to compiler-rt that
manages unsafe stack allocation/deallocation for each thread.

Original patch by Volodymyr Kuznetsov and others at the Dependable Systems
Lab at EPFL; updates and upstreaming by myself.

Differential Revision: http://reviews.llvm.org/D6096

llvm-svn: 239763