Use it instead of defining IN_RTLD by base sources that want P_OSREL_
defines in userspace, but are not rtld.
This allows to remove abuse of IN_RTLD from userspace.
Reviewed by: dchagin, markj, imp
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D38585