pl/math: Add scalar atan and set fenv in Neon atan

The simplest way to set fenv in Neon atan is by using a scalar
fallback for under/overflow cases, however this routine did not have a
scalar counterpart so we add a new one, based on the same algorithm
and polynomial as the vector variants, and accurate to 2.5 ULP. This
is now used as the fallback for all lanes, when any lane of the Neon
input is special.