All Arch Linux packages have had their files in the /lib directory moved to /usr/lib and now /lib is a symlink to usr/lib. When performing this update, pacman will likely identify a conflict in the /lib directory. In the simplest case, this is worked around by doing:
pacman -Syu --ignore glibc
pacman -Su
Never use --force during this update.
If either of this steps does not work (e.g. due to dependency version issues, file conflicts in /lib), refer to this guide for more detailed instructions on performing this upgrade.