Enable WASI feature, enhance security and add SGX sample (#142)

Change emcc to clang
Refine interpreter to improve perforamnce

core/iwasm/app-samples/hello-world/build.sh

@@ -1,6 +1,10 @@
 # Copyright (C) 2019 Intel Corporation.  All rights reserved.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-emcc -g -O3 *.c -s WASM=1 -s SIDE_MODULE=1 -s ASSERTIONS=1 -s STACK_OVERFLOW_CHECK=2 \
-                -s TOTAL_MEMORY=65536 -s TOTAL_STACK=4096 -o test.wasm
+clang-8 --target=wasm32 -O3 \
+        -z stack-size=4096 -Wl,--initial-memory=65536 \
+        -Wl,--allow-undefined, \
+        -Wl,--export=main, \
+        -Wl,--no-threads,--strip-all,--no-entry \
+        -nostdlib -o test.wasm *.c
 #./jeffdump -o test_wasm.h -n wasm_test_file test.wasm