[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.