sys/_types.h: define __(u)int(f)ptr_t for CHERI

On pure-capability ABIs, uintptr_t and variants are capabilities and
defined to the new primative type __uintcap_t and variants. This is
required to allow pointers (capabilities) to round trip through
uintptr_t as required by the C standard.

Reviewed by: kib, markj
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D53819