build: move target-independent hw/ objects to nested Makefile.objs

build: move target-independent hw/ objects to nested Makefile.objs

This patch starts converting the hw/ directory.  Some files in hw/
are compiled once, some twice (32-/64-bit), some once per target.
Each category is moved in a separate patch.

After this patch, the files that are compiled once will show the
same hierarchy in the build tree as they do in the source tree,
for example hw/qdev.o instead of just qdev.o.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>