lld: ignore the `-search_paths_first` option on MachO

ld64 provides the `-search_path_firsts` which will search each path in
the library search path order for both `lib[name].dylib`, `lib[name].a`
before moving on (searching all paths for the dylib and then falling
back to the static library if a shared library was not found).

This option has been the default for a long time, but the command line
flag still exists.  Ignore it for compatibility.