systemd: enable lto

Gentoo disabled systemd's preference for link-time-optimization because
it consumes a far amount of CPU and distcc doesn't run the linking step
remotely. This change alone shaves some 19MB from the uncompressed /usr
filesystem. That seems like a solid reason to leave it enabled.
systemd: run ldconfig on every boot
Merge pull request #820 from marineam/lto

systemd: enable lto, always run ldconfig