Change the samantic of struct link_map l_addr member.

It previously returned the object map base address, while all other
ELF operating systems return load offset, i.e. the difference between
map base and the link base.

Explain the meaning of the field in the man page.

Stop filling the mips-only l_offs member, which is apparently unused.

PR: 246561
Requested by: Damjan Jovanovic <damjan.jov@gmail.com>
Reviewed by: emaste, jhb, cem (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24918