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/config.h

@@ -81,6 +81,10 @@
 #define WASM_ENABLE_LAZY_JIT 0
 #endif
 
+#ifndef WASM_LAZY_JIT_COMPILE_THREAD_NUM
+#define WASM_LAZY_JIT_COMPILE_THREAD_NUM 4
+#endif
+
 #if (WASM_ENABLE_AOT == 0) && (WASM_ENABLE_JIT != 0)
 /* LazyJIT or MCJIT can only be enabled when AOT is enabled */
 #undef WASM_ENABLE_JIT