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

[Frank Ch. Eigler]

Roland McGrath <roland@redhat.com> writes:

> I don't think that flies so well.  The bugs we hit in older
> compilers were location lists that looked fine but were incorrect
> for the prologue regions.  There isn't any automatic way to identify
> those location lists as bad.

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.)

- FChE