exterr: allow exterr to fit pointers on CHERI targets

Switch to uint64ptr_t which is a uint64_t on traditional architectures
and a uintptr_t on CHERI architectures. This has no ABI impact on
non-CHERI kernels.

Fix truncation of 64-bit values on 32-bit kernels.

Reviewed by: kib
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D58056