Simplify `ld.lld --help` message.

Previously, we printed out two lines of help messages for `--foo bar`
and `--foo=bar` like this:

  --soname=<value>        Set DT_SONAME
  --soname <value>        Set DT_SONAME
  --sort-section=<value>  Specifies sections sorting rule when linkerscript is used
  --sort-section <value>  Specifies sections sorting rule when linkerscript is used

This change eliminates duplicate lines that doesn't contain `=` for such
options like this.

  --soname=<value>        Set DT_SONAME
  --sort-section=<value>  Specifies sections sorting rule when linkerscript is used

Differential Revision: https://reviews.llvm.org/D47558

llvm-svn: 333596