linux-user: Fix invalid TARGET_ABI_BITS usage on ppc hosts

When trying to evaluate the size of the _host_ type size for olddev_t,
we need to expose the host's pointer size, not the guest pointer size.

This usage got introduced accidently in commit b754e4fc1.

Fix things by not using TARGET_.*, but rather use host sizeof()
information, which gives us the correct size.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
