alarm/devtools-alarm: Remove setarch from a call to mkarchroot.

setarch does not support ARM architectures. Without this patch,
archbuild fails with this error:

    setarch: armv6h: Unrecognized architecture

This patch is similar in intent to the first hunk of
0004-arch-nspawn-arm-fix.patch.
Merge pull request #721 from paraxor/master

alarm/devtools-alarm: Remove setarch from a call to mkarchroot.