Restore the binary compatibility for link_map l_addr.

Keep link_map l_addr binary layout compatible, rename l_addr to l_base
where rtld returns map base. Provide relocbase in newly added l_addr.

This effectively reverts the patch to the initial version of D24918.

Reported by: antoine (portmgr)
Reviewed by: jhb, markj
Tested by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24946