[CMake] Fix building unit tests on Darwin

Summary:
There are a number of issues with unit tests on Darwin. These patches address the following:
* Unit tests should be passed -arch (-m32/-m64 isn't sufficient)
* Unit tests should be passed ${DARWIN_osx_CFLAGS} because they're being built for OS X
* Test architectures should be filtered based on base system capabilities (i.e. don't try running x86_64h tests on pre-haswell hardware).

Reviewers: bogner, filcab, kubabrecka

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12174

llvm-svn: 245580