objtool: Group headers to check in a single list

In order to support multiple architectures and potentially different
sets of headers to compare against their kernel equivalent, it is
simpler to have all headers to check in a single list.

Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Julien Thierry <jthierry@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>