When installing from the AUR, the building of cross-mipsel-linux-gnu-gcc fails and the whole installation subsequently fails too.
As mentioned in the comments on the AUR page of the package, the error occurs because the flag "-Werror=format-security" was added to the default flags of makepkg in a recent pacman release.
Removing it from the Cflags in /etc/makepkg.conf solves the issue.
The way I understand it the problem can't be fixed here, so if deemed appropriate it could be mentioned in the readme along with the workaround.