Adding a new option to CMake to disable C++ atexit on llvm-shlib.

Summary:
This is desirable for WebKit and other clients of the llvm-shlib because C++ exit time destructors have a tendency to crash when invoked from multi-threaded applications.

Ideally this option will be temporary, because the ideal fix is to just not have exit time destructors.

Reviewers: chapuni, ributzka

Reviewed By: ributzka

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6572