pl/math: Reduce order of single-precision tan polynomial

For both vector and scalar routines we reduce the order from 6 to
5. For vector routines, this requires reducing RangeVal as for large
values the tan polynomial is not quite accurate enough. However the
cotan polynomial is used in the inaccurate region in the scalar
routine, so this does not need to change.

Accuracy of scalar routine is unchanged. Accuracy in both vector
routines is now 3.45 ULP, with the same worst-case.