Makefile.target

In commit 4d330cee a new hostdep.h file was added, with the intent
that host architectures which needed one could provide it, and the
build system would automatically fall back to a generic version if
there was no version for the host architecture. Although this works,
it has a flaw: if a subsequent commit switches an architecture from
"uses generic/hostdep.h" to "uses its own hostdep.h" nothing in the
makefile dependencies notices this and so doing a rebuild without
a manual 'make clean' will fail.

So we drop the idea of having a 'generic' version in favour of
every architecture we support having its own hostdep.h, even if
it doesn't have anything in it. (There are only thirteen of these.)

If the dependency files claim that an object file depends on a
nonexistent file, our dependency system means that make will
rebuild the object file, and regenerate the dependencies in
the process. So moving between trees prior to this commit and
trees after this commit works without requiring a 'make clean'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>