Require trivial destructor in allocate_on_obstack

This patch makes allocate_on_obstack a little bit safer, by enforcing
the rule that objects allocated on an obstack must have a trivial
destructor.

The static assert is done in a method -- doing it inside the class
itself won't work because the class is incomplete at that point.

gdb/block.h

@@ -105,7 +105,7 @@ struct blockranges
    This implies that within the body of one function
    the blocks appear in the order of a depth-first tree walk.  */
 
-struct block : public allocate_on_obstack
+struct block : public allocate_on_obstack<block>
 {
   /* Return this block's start address.  */
   CORE_ADDR start () const