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

Change emcc to clang
Refine interpreter to improve perforamnce

test-tools/toolchain/sysroot/include/stdio.h

@@ -10,6 +10,10 @@
 extern "C" {
 #endif
 
+#ifndef NULL
+#  define NULL ((void*) 0)
+#endif
+
 typedef unsigned int size_t;
 
 int printf(const char *format, ...);