mender-workarounds.bbclass: set PACKAGE_ARCH for mender-client

The mender-client recipe in meta-mender-core creates a
device_type file containing ${MENDER_DEVICE_TYPE},
which defaults to ${MACHINE}. However, the PACKAGE_ARCH
for the recipe is not machine-specific, so different
machines with the same CPU architecture can end up with
the same device_type file (one or the other).

Work around this by adding an override in mender-workarounds
to make the package machine-specific.

Signed-off-by: Matt Madison <matt@madison.systems>