Place EH labels on the permanent obstack. Fixes PR 18314.

Background:
Originally, the EH labels were placed on the permanent obstack, which
could end up using a lot of memory (for heavy inlining) since inlined
labels also needed to be permanent as a result of this.

This was changed in

   Wed Dec  9 09:12:40 1998  Andrew MacLeod  <amacleod@cygnus.com>

	* except.h (struct handler_info): Add handler_number field.
	* except.c (gen_exception_label): EH labels no longer need to be
	on the permanent obstack.
	(get_new_handler): Set the label number field.
	(output_exception_table_entry): Regenerate handler label reference
	from the label number field.
	(init_eh): Remove a blank line.
	* integrate.c (get_label_from_map): Labels no longer need to be
	on the permanent obstack.

by using the label numbers instead of the label structures in most cases.
The operative word here is "most" cases. Addresses to the EH RTX was still
used in (at least) flow.c, that now used freed memory. Oops.

For this to happen, the freed address of the RTX representing a EH label
must be reused for a new label that is located in dead code. delete_block()
will then see that this RTX is mentioned in the EH table, and (incorrectly)
remove the exception handler.

This might be seen when, for example, compiling
src/gnu/dist/groff/src/roff/troff/node.cc for m68k.

gnu/dist/toolchain/gcc/except.c

@@ -617,7 +617,11 @@ rtx
 gen_exception_label ()
 {
   rtx lab;
+
+  push_obstacks_nochange ();
+  end_temporary_allocation ();
   lab = gen_label_rtx ();
+  pop_obstacks ();
   return lab;
 }
 

gnu/dist/toolchain/gcc/integrate.c

@@ -110,7 +110,12 @@ get_label_from_map (map, i)
   rtx x = map->label_map[i];
 
   if (x == NULL_RTX)
+    {
+      push_obstacks_nochange ();
+      end_temporary_allocation ();
       x = map->label_map[i] = gen_label_rtx();
+      pop_obstacks ();
+    }
 
   return x;
 }