arch.7: deprecate ILP32 and LP64 macros

With CHERI there now more options than ILP32 and LP64 so lack of
one does not imply the other. Encourage the use of SIZEOF_* macro
tests in their place.

Effort: CHERI upstreaming
Reviewed by: kib, markj, emaste
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D52820