stddef.h: centralize definition of offsetof()

Move to sys/_offsetof.h and use builtin_offsetof() instead of
offsetof to avoid reintroducing sys/cdefs.h polution in stddef.h.

This has the side effect of allowing sys/stddef.h to be included after
stddef.h which can happen in compatability headers.

Effort: CHERI upstreaming
Sponsored by: DARPA, AFRL
Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D55307