MAC: Use the current thread's user ABI to determine the layout of struct mac

This removes mac_label_copyin32() as mac_label_copyin() can now handle
both native and 32-bit struct mac objects.

Reviewed by: olce, brooks
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D53755