Fix powl, cpow, cpowf, and cpowl imports from OpenBSD

This is a follow-up to r336299.

• lib/msun/Makefile: . Remove polevll.c

• lib/msun/ld80/e_powl.c: . Copy contents of polevll.c to here. This is the only consumer of these functions. Make functions 'static inline'. . Make reducl a 'static inline' function.

• lib/msun/man/exp.3: . Remove BUGS section that no longer applies.

• lib/msun/src/math_private.h: . Remove prototypes of p1evll() and polevll()

• lib/msun/src/s_cpow.c:
• lib/msun/src/s_cpowf.c:
• lib/msun/src/s_cpowl.c . Use the CMPLX macro from either C99 or math_private.h (depends of compiler support) instead of the problematic use of complex I.

Submitted by: Steve Kargl <sgk@troutmask.apl.washington.edu>
PR: 229876
MFC after: 1 week