Make test work on windows

This is a hack, but "__asm__ __volatile__ ("mfence": : :"memory");" doesn't compile on windows and the way CBMC handles "__asm mfence" needs to be fixed. Currently "__asm mfence" is not recognised as a function by CBMC.

Author: polgreen

regression/goto-instrument/assembly_call_graph_test/main.c

@@ -1,8 +1,16 @@
+// This is a hack to make the test pass on windows. The way CBMC on
+// windows handles assembly code needs to be fixed.
+// CBMC doesn't recognise __asm mfence as a function.
+
+#ifndef __GNUC__
+void __asm_mfence();
+#endif
+
 void* thr(void * arg) {
 #ifdef __GNUC__
 __asm__ __volatile__ ("mfence": : :"memory");
-#elif defined(_MSC_VER)
-__asm mfence;
+#else
+__asm_mfence();
 #endif
 }