System.File_IO.Open calls full_name() for Create.  Use getcwd() approach
instead of realpath() since the latter fails if the file doesn't exist.

Index: gcc/ada/cstreams.c
--- gcc/ada/cstreams.c.orig
+++ gcc/ada/cstreams.c
@@ -190,7 +190,7 @@ __gnat_full_name (char *nam, char *buffer)
 	  *p = '\\';
     }
 
-#elif defined (__FreeBSD__) || defined (__DragonFly__) || defined (__OpenBSD__)
+#elif defined (__FreeBSD__) || defined (__DragonFly__)
 
   /* Use realpath function which resolves links and references to . and ..
      on those Unix systems that support it. Note that GNU/Linux provides it but