sys.mk: Improve ports compatibility hack guard

Back in 9ca543438039 and include of bsd.cpu.mk in sys.mk was added as a
hack for ports Makefiles. This works fine in general (for values of
fine that include not being able to use MK_ variables in bsd.cpu.mk),
but has surprising effects on MacOS with a case insensitive filesystems.
There we build things in paths like share/docs/IPv6 where
../../Mk/bsd.port.mk matches share/mk/bsd.port.mk.

Fix this issue by checking for ../../Mk/Uses which seems highly unlikely
to match something in share/mk.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D49232