CHERI: declare fueptr and suptr

These should replace fueword and suword when manipulating pointers in
memory. On CHERI targets they will be implemented using capability
aware instructions and otherwise they are defined to fueword and suword.

Reviewed by: kib, markj
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D57664