kbuild: refactor final link of sparc32

sparc32 uses an additional final link to support btfix.
Introduce a new set of exported variables in the top-level Makefile
to make the extra linking step simpler.

sparc32 has hardcoded knowledge of kallsyms support. This fix
include support for EXTRA_KALLSYM_PASS=1.
The ugly part is that it is hardcoded in the arch/sparc/boot
Makefile.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>