sys/_types.h: centralize vm_offset_t and vm_size_t definitions

Use ptraddr_t to define vm_offset_t and size_t for vm_size_t
rather than per-pointer-size definitions.

Reviewed by: kib, markj
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D53818