add support for Visual Studio style inline assembler

Author: kroening

regression/goto-instrument/assembly_call_graph_test/main.c

@@ -1,16 +1,10 @@
-// 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");
+  // gcc/clang syntax
+  __asm__ __volatile__("mfence" : : : "memory");
 #else
-__asm_mfence();
+  // this is Visual Studio syntax
+  __asm mfence;
 #endif
 }