Fix "objstack" typo

I noticed some comments that mentions "objstack".  The type is
actually "obstack".  This patch fixes the typos.

gdb/dwarf2/read.c

@@ -6892,7 +6892,7 @@ dwarf2_full_name (const char *name, struct die_info *die, struct dwarf2_cu *cu)
 
 /* Construct a physname for the given DIE in CU.  NAME may either be
    from a previous call to dwarf2_name or NULL.  The result will be
-   allocated on the objfile_objstack or NULL if the DIE does not have a
+   allocated on the objfile_obstack or NULL if the DIE does not have a
    name.
 
    The output string will be canonicalized (if C++).  */

gdb/gdbtypes.c

@@ -2920,9 +2920,9 @@ type::remove_dyn_prop (dynamic_prop_node_kind kind)
       if (curr_node->prop_kind == kind)
 	{
 	  /* Update the linked list but don't free anything.
-	     The property was allocated on objstack and it is not known
+	     The property was allocated on obstack and it is not known
 	     if we are on top of it.  Nevertheless, everything is released
-	     when the complete objstack is freed.  */
+	     when the complete obstack is freed.  */
 	  if (NULL == prev_node)
 	    this->main_type->dyn_prop_list = curr_node->next;
 	  else