[LibFuzzer] Reimplement how the optional user functions are called.

The motivation for this change is to fix linking issues on OSX.
However this only partially fixes linking issues (the uninstrumented
tests and a few others  won't succesfully link yet).

This change introduces a struct of function pointers
(``fuzzer::ExternalFuntions``) which when initialised will point to the
optional functions if they are available.  Currently these
``LLVMFuzzerInitialize`` and ``LLVMFuzzerCustomMutator`` functions.

Two implementations of ``fuzzer::ExternalFunctions`` constructor are
provided one for Linux and one for OSX.

The OSX implementation uses ``dlsym()`` because the prior implementation
using weak symbols does not work unless the additional flags are passed
to the linker.

The Linux implementation continues to use weak symbols because the
``dlsym()`` approach does not work unless additional flags are passed
to the linker.

Differential Revision: http://reviews.llvm.org/D20741

llvm-svn: 271491