cats and netwinder can share a common footbridge_intr.h file, so install and
share a common file.

Enable the use of a seperate statclock.

Currently statclock runs at 64hz, maybe it should be faster or slower, I did
try it being the same as hz, but that just made it look like we spent 10% of
time

Enable the use of a seperate statclock.

Currently statclock runs at 64hz, maybe it should be faster or slower, I did
try it being the same as hz, but that just made it look like we spent 10% of
time handling interrupts, rather than the 3% that this gives.

Also fix the IPL_LEVELS for netwinder.

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Use <arm/arm32/psl.h> rather than <machine/psl.h>.

Change _ARM32* to _NETWINDER* Include <machine/psl.h> to get psl
definitions.

Populate the netwinder include directory.