build-sys: Use slim LTO objects if possible

While 235c6e62 gets LTO builds running again, it goes back to using fat
LTO objects instead of using gcc-{ar,nm,ranlib}. Building these fat
objects takes significantly more time.

Use the suggested solution and look for gcc-{ar,nm,ranlib}, which launch
the binutils tools with the appropriate --plugin parameter. GCC versions
that do not ship these should either default to -ffat-lto-objects or do
not support LTO at all.

On another note, nm from binutils 2.25 seems to be smart enough to load
the LTO plugin when needed without having to specify --plugin.