Constify linetables

Linetables no longer change after they are created.  This patch
applies const to them.

Note there is one hack to cast away const in mdebugread.c.  This code
allocates a linetable using 'malloc', then later copies it to the
obstack.  While this could be cleaned up, I chose not to do so because
I have no way of testing it.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/disasm.c

@@ -573,7 +573,7 @@ do_mixed_source_and_assembly_deprecated
 {
   int newlines = 0;
   int nlines;
-  struct linetable_entry *le;
+  const struct linetable_entry *le;
   struct deprecated_dis_line_entry *mle;
   struct symtab_and_line sal;
   int i;