move syscallno into Task

x86 and x86-64 have wildly disparate syscall numbering schemes, so
anything dealing with syscall numbers is going to have to acquire some
32/64-bit awareness.  This patch is merely the first step in doing so.
Task seems like the best place to put the interface, even if the actual
implementation might not live on Task in the future.

I don't know that the changes to `Event` are the best, but I didn't see a good way of getting a `Task` handle at that point.