mkarchroot: use bind mount instead of symlink for /dev/ptmx

We need /dev/ptmx -> /dev/pts/ptmx (for devpts -o newinstance)
Other way to do this thing is via bind mount (as said kernel doc[devpts.txt]).

This should be done in this way at least for /dev as devtmpfs in the chroot.
Since we can not touch /dev (devtmpfs), because devtmpfs is "singleton",
just use bind method and avoid interference.

Do it the same for both modes of /dev (tmpfs) and (devtmpfs) to keep it simple.

Currently devpts in chroot is not working without this when using /dev as devtmpfs,
this fixes this issue (opening /dev/ptmx, creates devices nodes on outside /dev/pts)

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>