This exposes more MCJIT options via the C API:

CodeModel: It's now possible to create an MCJIT instance with any CodeModel you like.  Previously it was only possible to
create an MCJIT that used CodeModel::JITDefault.

EnableFastISel: It's now possible to turn on the fast instruction selector.

The CodeModel option required some trickery.  The problem is that previously, we were ensuring future binary compatibility in
the MCJITCompilerOptions by mandating that the user bzero's the options struct and passes the sizeof() that he saw; the
bindings then bzero the remaining bits.  This works great but assumes that the bitwise zero equivalent of any field is a
sensible default value.

But this is not the case for LLVMCodeModel, or its internal equivalent, llvm::CodeModel::Model.  In both of those, the default
for a JIT is CodeModel::JITDefault (or LLVMCodeModelJITDefault), which is not bitwise zero.

Hence this change introduces LLVMInitializeMCJITCompilerOptions(), which will initialize the user's options struct with
defaults. The user will use this in the same way that they would have previously used memset() or bzero(). MCJITCAPITest.cpp
illustrates the change, as does the comment in ExecutionEngine.h.