parisc: Add runtime check to prevent PA2.0 kernels on PA1.x machines

If a 32-bit kernel was compiled for PA2.0 CPUs, it won't be able to run
on machines with PA1.x CPUs. Add a check and bail out early if a PA1.x
machine is detected.

Signed-off-by: Helge Deller <deller@gmx.de>