PowerPC: Fix modf/modff optimization return sign

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.