Where possible use --long-opt=value in lld man page

We intend to maintain compatibility with GNU ld, and in the GNU world
long options are conventionally specified as --long-option=value. For
whatever reason GNU ld.bfd accepts both --long-option value and
--long-option=value, but documents the former. Follow suit.

Some lld long options do not accept the = form; this is probably a bug
to be fixed (along with a man page update).

Reported by Ingo Schwarze, for --entry.

llvm-svn: 326459