Pass $(CROSS_COMPILE) or host GNU type through to upstream build rules.
debian/rules.real: Filter tools packages by host arch, not build arch
debian/rules.d/Makefile: Build the tools needed for headers_install in
a separate subdirectory
Martin Michlmayr found that userland builds fail if 'uname -m' reports
the native kernel architecture, and that's because we don't explicitly
set ARCH for the headers_install target. That's fine for most cases
of a 32-bit chroot on a 64-bit kernel because a single source
architecture covers both, but arm/arm64 is an exception.
This doesn't make actual cross-builds of userland tools work.
By passing both -C and -f options to make, we can separate makefiles
from the output directory without adding prefixes to all targets.
We can also reduce the 'clean' commands to little more than 'rm -rf'.
Ben Hutchings