[OCaml] Build a wrapper for LLVMGetFunctionAddress

Summary:
I've verified that the LLVMGetFunctionAddress is receiving the right
string, and calling down to the MCJIT implementation of
getFunctionAddress. However, getSymbolAddress does not seem to want to
cooperate with us, and returns 0 for a perfectly valid function. Why
LLVMGetFunctionAddress works via the C API, but not via the OCaml API
after having verified that the arguments are right stumps me.

Perhaps there is some name mangling going on when OCaml emits the IR? I
can't imagine why, but it's the only explanation I've been able to reach
so far.

Reviewers: whitequark

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6761