[OCaml] Expose Llvm_executionengine.ExecutionEngine.create_mcjit.
[OCaml] Unbreak Llvm_executionengine.initialize_native_target.

First, return true on success, as it is the OCaml convention.
Second, also initialize the native assembly printer, which is,
despite the name, required for MCJIT operation.

Since this function did not initialize the assembly printer earlier
and no function to initialize native assembly printer was available
elsewhere, it is safe to break its interface: it means that it
simply could not be used successfully before.