target-alpha: Move alarm to vm_clock

Basing the alarm off the rtc_clock was silly.  It leads to horrible
spinning in the guest after being suspended and resumed, as it tries
to catch up with lost ticks.

This requires adding an accessor for reading the vm_clock too.

Signed-off-by: Richard Henderson <rth@twiddle.net>
pc-bios: Update palcode-clipper

Update image to c87a92639b28ac42bc8f6c67443543b405dc479b,
incorporating changes for vm_time.

Signed-off-by: Richard Henderson <rth@twiddle.net>