The new native-xtools uses 'make toolchain' so overriding DESTDIR
as a make argument may interfere with WORLDTMP handling.
The target also does a 'mkdir -p ${NXBDESTDIR}/usr', so we should
be modifying that rather than DESTDIR.
Note this causes the native-xtools binaries to be installed in
NANO_WORLDDIR/usr NANO_WORLDDIR/bin rather than NANO_WORLDDIR/nxb-bin/usr
and NANO_WORLDDIR/nxb-bin/bin. This was the case before this change
as well.