The use of the math library powl was blacklisted on Mac OSX
(numpygh-8318) because powl(0, y) would trigger a divide-by-zero
warning even when y > 0 (issue numpygh-8307).

This change creates npy_powl on OSX that avoids the spurious
warning by not calling powl when x = 0 and y > 0.

The change in npy_math_internal.h.src moves the handling
of pow out of a template repeat-loop, so powl and powf
are handled individually.  This allows the creation of
npy_powl to be specialized when NPY_OS_DARWIN is defined.

Closes numpygh-8608.