Add Darwin blacklist to GCC torture suite's CMakeLists.txt

Some of these tests use features which are not supported on Darwin. (E.g.
aliases)

As a result of this, there are some unhappy Darwin test suite bots. So, let's
just blacklist those tests when we're targeting Darwin.

Example of unhappy bot:
http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-x86_64-O0-g/
1 file changed