Use C.UTF-8 locale

Now that the glibc package ships with C.UTF-8 included, there's no need to generate en_US.UTF-8 just to ensure a UTF-8 locale is used. mkarchiso also doesn't need to use the non-UTF-8 C locale.

TODO:

• Set LANG=C.UTF-8 in configs/baseline/airootfs/etc/locale.conf and configs/releng/airootfs/etc/locale.conf,
• Remove configs/releng/airootfs/etc/pacman.d/hooks/40-locale-gen.hook,
• Replace export LC_ALL="C" with export LC_ALL="C.UTF-8" in mkarchiso.