[safestack] Add option for non-TLS unsafe stack pointer.

This patch adds an option, -safe-stack-no-tls, for using normal
storage instead of thread-local storage for the unsafe stack pointer.
This can be useful when SafeStack is applied to an operating system
kernel.

http://reviews.llvm.org/D15673

Patch by Michael LeMay.

llvm-svn: 256221