kbuild: Add target debug_kallsyms

From: Keith Owens <kaos@ocs.com.au>

Make it easier to generate maps for debugging kallsyms problems.
debug_kallsyms is only a debugging target so no help or silent mode.

Signed-off-by: Keith Owens <kaos@ocs.com.au>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>