Allow CMAKE_OSX_ARCHITECTURES to build fat library.

The CMAKE_OSX_ARCHITECTURES CMake variable allows users to build
universal fat libraries that contain both i386 and x86_64 code.  These
changes allow this build by having the z_Linux_asm.s file detect the architecture
itself instead of receiving it through the build system.

Also, there was a LIBOMP_OSX_ARCHITECTURES CMake variable added to allow
people to only build libomp as a fat library and not the entire LLVM/Clang system.
http://lists.cs.uiuc.edu/pipermail/openmp-dev/2015-May/000626.html

llvm-svn: 238566