common: make sure symbols-dummy.o gets rebuilt when needed

common: make sure symbols-dummy.o gets rebuilt when needed

The per-arch top level make files don't record any dependencies for the
file, so its mere existence is enough for make to consider it up-to-
date. As of ab3e5f5ff9 ("xsplice, symbols: Implement fast symbol names
-> virtual addresses lookup") the file, however, depends on the
FAST_SYMBOL_LOOKUP config option, which may change between incremental
re-builds.

Use the $(extra-y) machinery to get the file built without an extra
recursion step into common/, but instead right when the other things in
that directory get built. Some makefile adjustments are necessary to
actually make this machinery work beyond the restricted set of place it
was used in before. Note however that an important restriction remains:
$(extra-y) may not overlap $(obj-y) or $(obj-bin-y).

Take the opportunity and also make the gendep invocation cover both
$(obj-bin-y) and $(extra-y), even if this is not directly related here.
I should have included them right away in 8b6ef9c152 ("compat: enforce
distinguishable file names in symbol table").

Reported-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>