LinuxKPI: add __struct_size

Use builtin_object_size() by default.
If builtin_dynamic_object_size() is available use that instead.

I would hope that in a future version we can remove the conditional
checks but in order to be able to MFC this without checking all
compiler versions simply keep it this way for now.

Sponsored by: The FreeBSD Foundation (initially)
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D52077