TODO(morello): cheri: fix posix timers

TODO(morello): cheri: fix posix timers

We need to distinguish timerids that are small integers returned by
the kernel and timerids that are pointers to struct timer. The existing
pointer tagging does not work for CHERI because of the pointer shift.

Simply use the top bit without shift to tag pointers. This still relies
on the top byte ignore of aarch64 (the top byte does not affect the
capability representation) and that pointers are not tagged for other
reasons (like HWASAN).

TODO: this is morello specific and does not work for generic cheri.