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

Change emcc to clang
Refine interpreter to improve perforamnce

samples/simple/src/main.c

@@ -2,4 +2,5 @@ extern void iwasm_main();
 int main(int argc, char *argv[])
 {
     iwasm_main(argc, argv);
+    return 0;
 }