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/compilation/aot_emit_exception.c

@@ -83,9 +83,9 @@ aot_emit_exception(AOTCompContext *comp_ctx, AOTFuncContext *func_ctx,
         }
         else {
             /* Create LLVM function with external function pointer */
-            if (!(func = LLVMGetNamedFunction(comp_ctx->module,
+            if (!(func = LLVMGetNamedFunction(func_ctx->module,
                                               "aot_set_exception_with_id"))
-                && !(func = LLVMAddFunction(comp_ctx->module,
+                && !(func = LLVMAddFunction(func_ctx->module,
                                             "aot_set_exception_with_id",
                                             func_type))) {
                 aot_set_last_error("add LLVM function failed.");