Add flow graph printing into a .cfg file with flag --print-flow-graph-file.

vm/os_macos.cc

Index: vm/os_macos.cc
===================================================================
--- vm/os_macos.cc	(revision 8145)
+++ vm/os_macos.cc	(working copy)
@@ -150,6 +150,14 @@
 }
 
 
+void OS::FPrint(FILE* stream, const char* format, ...) {
+  va_list args;
+  va_start(args, format);
+  VFPrint(stream, format, args);
+  va_end(args);
+}
+
+
 void OS::VFPrint(FILE* stream, const char* format, va_list args) {
   vfprintf(stream, format, args);
   fflush(stream);
@@ -178,6 +186,13 @@
 }
 
 
+FILE* OS::FOpen(const char* path, const char* mode) {
+  FILE* file = fopen(path, mode);
+  if (file == NULL) return NULL;
+  return file;
+}
+
+
 void OS::InitOnce() {
   // TODO(5411554): For now we check that initonce is called only once,
   // Once there is more formal mechanism to call InitOnce we can move