Spinlock code

Anybody feel like taking a shot at implementing spinlocks?  I have been
looking through the Postgres-8.2 code and I talked a bit with Bruce
Momjian about their implementation and I think we could lift a good
amount of code to get portable spinlocks from them.

See:

postgresql-8.2.0/src/include/storage/s_lock.h

And also see how they are used in:

postgresql-8.2.0/src/backend/storage/lmgr/lwlock.c

PostgreSQL is completely BSD-licensed, so it is a good place to steal
code from.

-Rasmus