[llvm-c] Expose getFunctionAddress, as a modern alternative

Summary:
The OCaml bindings use getPointerToGlobal as the primary way to get a
handle on functions to run. This function is broken and deprecated, and
we want to transition to the modern getFunctionAddress alternative. This
is a two step process: first, there needs to be a C wrapper that OCaml
can leverage. So, build LLVMGetFunctionAddress with a test.

Second, there needs to be an OCaml interface to the new
LLVMGetFunctionAddress C function. This will be done in another patch.

Reviewers: whitequark

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6760