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

product-mini/platforms/linux/build_jit.sh

@@ -5,8 +5,8 @@
 
 rm -fr build && mkdir build
 cd build
-# Build With LazyJIT
-# cmake .. -DWAMR_BUILD_JIT=1 -DWAMR_BUILD_LAZY_JIT=1
+# By default LazyJIT is enabled, to disable it:
+# cmake .. -DWAMR_BUILD_JIT=1 -DWAMR_BUILD_LAZY_JIT=0
 cmake .. -DWAMR_BUILD_JIT=1
-make
+make -j ${nproc}
 cd ..