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_compiler.h

@@ -345,8 +345,8 @@ check_type_compatible(uint8 src_type, uint8 dst_type)
         else {                                                              \
             char *func_name = #name;                                        \
             /* AOT mode, delcare the function */                            \
-            if (!(func = LLVMGetNamedFunction(comp_ctx->module, func_name)) \
-                && !(func = LLVMAddFunction(comp_ctx->module, func_name,    \
+            if (!(func = LLVMGetNamedFunction(func_ctx->module, func_name)) \
+                && !(func = LLVMAddFunction(func_ctx->module, func_name,    \
                                             func_type))) {                  \
                 aot_set_last_error("llvm add function failed.");            \
                 goto fail;                                                  \