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

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