Import WAMR Fast JIT (#1343)

Import WAMR Fast JIT which is a lightweight JIT with quick startup, small footprint,
relatively good performance (~40% to ~50% of LLVM JIT) and good portability.

Platforms supported: Linux, MacOS and Linux SGX.
Arch supported: x86-64.

doc/linux_sgx.md

@@ -11,12 +11,18 @@ After installing the dependencies, build the source code:
 ``` Bash
 source <SGX_SDK dir>/environment
 cd product-mini/platforms/linux-sgx/
-mkdir build
-cd build
+mkdir build && cd build
 cmake ..
 make
 ```
 
+By default the `fast interpreter` and `AOT` is enabled. If to enable `Fast JIT`, run:
+```Bash
+mkdir build && cd build
+cmake .. -DWAMR_BUILD_FAST_JIT=1
+make
+```
+
 This builds two libraries required by SGX application:
  - libvmlib.a for Enclave part
  - libvmlib_untrusted.a for App part