lld: add -z interpose support

-z interpose sets the DF_1_INTERPOSE flag, marking the object as an
interposer.

Committed upstream as LLVM r342239.

PR: 230604
Reported by: jbeich
Reviewed by: markj
Approved by: re (kib)
MFC after: 1 week
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17172