rusticl/api: Implement get_{device_and_}host_timer

rusticl/api: Implement get_{device_and_}host_timer

Use the get_timestamp as both the device_timestamp in
get_device_and_host_timer and host_timestamp in that
and get_host_timer.

Having eliminited most other clock sources, discussions
on previous versions have concluded it's best to use the
same timer as the 'host_timestamp' since the main requirements
are that it must be one that's a time seen by the device and
that it's very closely coupled.

Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23639>