objtool: Make sync-check consider the target architecture

Do not take into account outdated headers unrelated to the build of the
current architecture.

[ jpoimboe: use $SRCARCH directly ]

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