Probe on function entry rather than the end of the prolog (s390x only)

[Roland McGrath]

> Too bad.  OK, then let's gather some data about compiler versions and
> architectures.  If we find a reasonable and documentable dividing line
> between working and not working, then we can make systemtap assume the
> former.  (We could retain a discreet switch such as an environment
> variable to activate the prologue code for people stuck with
> non-working systems.)

Check with Jakub and Alex, to dig up which versions first got the fixes.