lld: introduce basic man page

Upstream lld has no man page. Introduce a basic one for FreeBSD based on
ld.lld --help, with a brief introduction and additional detail for some
options.

We'll continue refining this in FreeBSD, and then submit it upstream once
the first round of edits are complete.

Submitted by: krion, Arshan Khanifar, emaste, bjk
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13813