PowerPC: modf optimization fix

This patch fix the 3c0265394d9ffedff2b0de508602dc52e077ce5c commits
by correctly setting minimum architecture for modf PPC optimization
to power5+ instead of power5 (since only on power5+ round/ceil will
be inline to inline assembly).
Fix csin, csinh, ccos, ccosh missing underflows (bug 15405).
Fix catan, catanh missing underflows (bug 15406).