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/iwasm/aot/aot_runtime.c

@@ -1419,6 +1419,17 @@ aot_call_function(WASMExecEnv *exec_env, AOTFunctionInstance *function,
     }
     argc = func_type->param_cell_num;
 
+#if WASM_ENABLE_LAZY_JIT != 0
+    if (!function->u.func.func_ptr) {
+        AOTModule *aot_module = (AOTModule *)module_inst->aot_module.ptr;
+        if (!(function->u.func.func_ptr =
+                  aot_lookup_orcjit_func(aot_module->comp_ctx->orc_lazyjit,
+                                         module_inst, function->func_index))) {
+            return false;
+        }
+    }
+#endif
+
     /* set thread handle and stack boundary */
     wasm_exec_env_set_thread_info(exec_env);
 
@@ -2300,6 +2311,15 @@ aot_call_indirect(WASMExecEnv *exec_env, uint32 tbl_idx, uint32 table_elem_idx,
     func_type_idx = func_type_indexes[func_idx];
     func_type = aot_module->func_types[func_type_idx];
 
+#if WASM_ENABLE_LAZY_JIT != 0
+    if (func_idx >= aot_module->import_func_count && !func_ptrs[func_idx]) {
+        if (!(func_ptr = aot_lookup_orcjit_func(
+                  aot_module->comp_ctx->orc_lazyjit, module_inst, func_idx))) {
+            return false;
+        }
+    }
+#endif
+
     if (!(func_ptr = func_ptrs[func_idx])) {
         bh_assert(func_idx < aot_module->import_func_count);
         import_func = aot_module->import_funcs + func_idx;