Make the __bitcount*() functions unconditionally available, by moving

Make the __bitcount*() functions unconditionally available, by moving
them out of the #if _BSD_VISIBLE block.  Other headers may depend on
__bitcount().  The dependencies can be a header not specified by
POSIX, and then namespace restrictions by _XOPEN_SOURCE are not
applicable, as it was reported.  Or, we might grow an implementation
of some POSIX facility using __bitcount(), which also should work.

Reported by:	Jason Schulz <schulz.j@gmail.com>
Discussed with:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks