msun: add ld80/ld128 powl, cpow, cpowf, cpowl from openbsd

This corresponds to the latest status (hasn't changed in 9+
years) from openbsd of ld80/ld128 powl, and source cpowf, cpow,
cpowl (the complex power functions for float complex, double
complex, and long double complex) which are required for C99
compliance and were missing from FreeBSD. Also required for
some numerical codes using complex numbered Hamiltonians.

Thanks to jhb for tracking down the issue with making
weak_reference compile on powerpc.

When asked to review, bde said "I don't like it" - but
provided no actionable feedback or superior implementations.

Discussed with: jhb
Submitted by: jmd
Differential Revision: https://reviews.freebsd.org/D15919