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

[Frank Ch. Eigler]

I'm game for going prologue-free by default, agan subject to more
testing.  I will check out RHEL4 and RHEL5 on i386 and x86-64.  If
others can produce reports of the "installcheck" testsuite (and
perhaps some hand-run eye-observed tests) for their own favorite
platforms, we can flip the default.

- FChE