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

build-scripts/config_common.cmake

@@ -87,7 +87,9 @@ endif ()
 if (WAMR_BUILD_JIT EQUAL 1)
   if (WAMR_BUILD_AOT EQUAL 1)
     add_definitions("-DWASM_ENABLE_JIT=1")
-    if (WAMR_BUILD_LAZY_JIT EQUAL 1)
+    if (NOT WAMR_BUILD_LAZY_JIT EQUAL 0)
+      # Enable Lazy JIT by default
+      set (WAMR_BUILD_LAZY_JIT 1)
       add_definitions("-DWASM_ENABLE_LAZY_JIT=1")
     endif ()
     if (NOT DEFINED LLVM_DIR)
@@ -128,11 +130,10 @@ else ()
   message ("     WAMR AOT disabled")
 endif ()
 if (WAMR_BUILD_JIT EQUAL 1)
-  message ("     WAMR JIT enabled")
   if (WAMR_BUILD_LAZY_JIT EQUAL 1)
-    message ("     WAMR LazyJIT enabled")
+    message ("     WAMR Lazy JIT enabled")
   else ()
-    message ("     WAMR LazyJIT disabled")
+    message ("     WAMR MC JIT enabled")
   endif ()
 else ()
   message ("     WAMR JIT disabled")