Allow sysctl hw.machine/hw.machine_arch in capability mode

There's no harm in reading strings like 'amd64'.

Reviewed by: emaste, manu
Sponsored by: https://www.patreon.com/valpackett
Differential Revision: https://reviews.freebsd.org/D28703

(cherry picked from commit 4a1c4de232af202aa5fa02c7adaa97174997ae61)