arch.7: decouple long and pointer sizes

Don't claim void* or (u)intptr_t are the same size as long. This isn't
true for CHERI.

Effort: CHERI upstream
Reviewed by: kib, markj, jhb, emaste
Sponsored by: Innovate UK, DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D52821