Initial libjit skeleton.

Not currently generating instructions, but most of the flow is right up to
that point. A lot of work required to refactor the emitter, but wanted to
get this checked in.

tools/xenia-test/xenia-test.cc

@@ -120,7 +120,7 @@ int run_test(string& src_file_path) {
   memory = xe_memory_create(memory_options);
   XEEXPECTNOTNULL(memory);
 
-  //backend = shared_ptr<Backend>(new xe::cpu::llvmbe::LLVMBackend());
+  backend_ = shared_ptr<Backend>(new xe::cpu::libjit::LibjitBackend());
 
   processor = shared_ptr<Processor>(new Processor(memory, backend));
   XEEXPECTZERO(processor->Setup());