Since clang 3.6.0 now implements the archetype 'freebsd_kprintf' for
attribute((format(...))), and the -fformat-extensions flag was
removed, introduce a new macro in bsd.sys.mk to choose the right variant
of compile flag for the used compiler, and use it.

Also add something similar to kern.mk, since including bsd.sys.mk from
that file will anger Warner. :-)

Note that bsd.sys.mk does not support the MK_FORMAT_EXTENSIONS knob used
in kern.mk, since that knob is only available in kern.opts.mk, not in
src.opts.mk. We might want to add it later, to more easily support
external compilers for building world (in particular, sys/boot).