Add ld.lld.1 man page, describing lld's ELF linker

One of the FreeBSD requirements for enabling lld as the system linker is
that it has a man page. Other FreeBSD developers and I created one using
lld's --help output as a starting point.  This is an expanded and
improved version of that.

This man page would benefit from additional content, but it provides
basic user-facing coverage of lld's options and serves as a good
starting point.

Differential Revision:	https://reviews.llvm.org/D42963

llvm-svn: 324512