Disable build of LOCAL_MODULES for cross-builds by default.

WITHOUT_LOCAL_MODULES can be set to disable LOCAL_MODULES for native
builds. WITH_LOCAL_MODULES can be set to leave it enabled for cross
builds.

This does not use a knob in kern.opts.mk because the options framework
does not currently support options whose default varies on the build
type. I discussed a few options there with Warner (e.g. maybe having
a tri-state where the default value is "auto" and having Makefile.inc1
apply logic when MK_LOCAL_MODULES is set to "auto"), but Warner ok'd
this approach for now until a better solution is implemented.

Requested by: many
Reviewed by: imp (in person at EuroBSDCon)
Differential Revision: https://reviews.freebsd.org/D21608