objtool: Extricate sls from stack validation

Extricate sls functionality from validate_branch() so they can be
executed (or ported) independently from each other.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Link: https://lkml.kernel.org/r/2545c86ffa5f27497f0d0c542540ad4a4be3c5a5.1650300597.git.jpoimboe@redhat.com