This adds the app target arch to the list of the allowed arches
in seccomp. It also allows the native arch, because otherwise
xdg-app-helper can't run.
This means that if your userspace (i.e. xdg-app-helper) is built in
64bit mode, then 64bit apps can't run 32bit apps. However, if you're
running a 32bit app it is allowed to run both 32bit and 64bit apps.
Not ideal, but at least the common case is safer.
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