Add support for LLVM_USE_SANITIZER = SafeStack

[Valérian Rousset via llvm-commits]

Hello all,

Here is a trivial patch to allow LLVM to be build with SafeStack, via
LLVM_USE_SANITIZER=SafeStack. 

It does compile without issue, but some tests fail, mainly due to profiling with
instrumentation.
It fails with `undefined reference to `__safestack_unsafe_stack_ptr'` because it
has to link with the SafeStack runtime but it wasn't build. What is the best way
to do this? Mark the tests as unsupported or did I missed something?

Have a nice day.
-- 
tharvik
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