Refactor Orc JIT to enable lazy compilation (#974)

Refactor LLVM Orc JIT to actually enable the lazy compilation and speedup
the launching process:
  https://llvm.org/docs/ORCv2.html#laziness

Main modifications:
- Create LLVM module for each wasm function, wrap it with thread safe module
  so that the modules can be compiled parallelly
- Lookup function from aot module instance's func_ptrs but not directly call the
  function to decouple the module relationship
- Compile the function when it is first called and hasn't been compiled
- Create threads to pre-compile the WASM functions parallelly when loading
- Set Lazy JIT as default, update document and build/test scripts

core/shared/platform/common/posix/posix_thread.c

@@ -32,7 +32,9 @@ os_thread_wrapper(void *arg)
     os_signal_handler handler = targ->signal_handler;
 #endif
 
+#if 0
     os_printf("THREAD CREATED %jx\n", (uintmax_t)(uintptr_t)pthread_self());
+#endif
     BH_FREE(targ);
 #ifdef OS_ENABLE_HW_BOUND_CHECK
     if (os_thread_signal_init(handler) != 0)