[LibFuzzer] Disable the ``fuzzer-trace-pc.test`` test on non-linux platforms.

On OSX this test sometimes fails due to the
``LLVMFuzzer-FullCoverageSetTest-TracePC`` program going over the
default 2GiB memory limit. This shouldn't be happening and needs
investigating. For now just disable the test so we can set up an
OSX buildbot.

Differential Revision: http://reviews.llvm.org/D21319

llvm-svn: 272696