Reduce diff between msun/src/e_pow.c and msun/src/e_powf.c.

Remove unnecessary casts, use integer literal constants instead of
floating point constants where possible, and introduce three const
static variables to hold 0.5, 0.25, and 1/3.

PR: 229420
Submitted by: Steve Kargl <sgk@troutmask.apl.washington.edu>
MFC after: 1 week