LaTeX writer: add \label's for all index keys, not just first

This change was originally added in coq project, but I am forwarding
it here at request of the patch author.

See coq/coq#12361 for the issue description.

sphinx/writers/latex.py

@@ -687,10 +687,10 @@ def _depart_signature_line(self, node: Element) -> None:
         self.body.append('}')
 
     def visit_desc_signature(self, node: Element) -> None:
+        hyper = ''
         if node.parent['objtype'] != 'describe' and node['ids']:
-            hyper = self.hypertarget(node['ids'][0])
-        else:
-            hyper = ''
+            for id in node['ids']:
+                hyper += self.hypertarget(id)
         self.body.append(hyper)
         if not self.in_desc_signature:
             self.in_desc_signature = True