rc: remove the 'archdep' script

It was unused since 405c3050f10, which removed iBCS support.

This also moves the 'linux' rc script slightly earlier, which
might help in some setups. The original version of this patch
moved it even more, before 'mountcritlocal', which would fixe
mount(8) errors due to missing /dev/shm in setups with entries
for /path/to/chroot/dev/shm without the "late" flag; however,
in the end 'kldxref' turned out to depend on 'mountcritlocal'
anyway.

Reported By: pstef
Reviewed By: dchagin
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D29590