[LibFuzzer]

Work around crashes in ``__sanitizer_malloc_hook()`` under Mac OSX.

Under Mac OSX we intercept calls to malloc before thread local
storage is initialised leading to a crash when accessing
``AllocTracer``. To workaround this ``AllocTracer`` is only accessed
in the hook under Linux. For symmetry ``__sanitizer_free_hook()``
is also modified in the same way.

To support this change a set of new macros
LIBFUZZER_LINUX and LIBFUZZER_APPLE has been defined which can be
used to check the target being compiled for.

Differential Revision: http://reviews.llvm.org/D20402

llvm-svn: 270145