This patch fix the optimized powerpc-fpu modf/modff implementation
when using in non-default rounding mode where the zero sign is not
as expected. It fixes the libm testsuite tests
modf_downward (0) == 0.00000000000000000000e+00
modf_downward (20) == 0.00000000000000000000e+00
modf_downward (21) == 0.00000000000000000000e+00
Where the sign returned was negative.
This patch fix the 3c0265394d 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).
