objtool: Skip ORC entry creation for non-text sections

Orc generation is only done for text sections, but some instructions
can be found in non-text sections (e.g. .discard.text sections).

Skip setting their orc sections since their whole sections will be
skipped for orc generation.

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