Migrate ie->ie_assign_cpu and associated code to use an int for CPU rather
than u_char.

Migrate post_filter to use an int for a CPU rather than u_char.

Change intr_event_bind() to use an int for CPU rather than u_char.

It touches the ppc, sparc64, arm and mips machdep code but it should
(hah!) be a no-op.

Tested:

• i386, AMD64 laptops

Reviewed by: jhb