gdb: Add new 'print nibbles' feature

Make an introduction of a new print setting that can be set by 'set
print nibbles [on|off]'.  The default value if OFF, which can be changed
by user manually.  Of course, 'show print nibbles' is also included in
the patch.

The new feature displays binary values by group, with four bits per
group.  The motivation for this work is to enhance the readability of
binary values.

Here's a GDB session before this patch is applied.
  (gdb) print var_a
  $1 = 1230
  (gdb) print/t var_a
  $2 = 10011001110

With this patch applied, we can use the new print setting to display the
new form of the binary values.
  (gdb) print var_a
  $1 = 1230
  (gdb) print/t var_a
  $2 = 10011001110
  (gdb) set print nibbles on
  (gdb) print/t var_a
  $3 = 0100 1100 1110

Tested on x86_64 openSUSE Tumbleweed.

gdb/language.h

@@ -282,6 +282,13 @@ struct language_defn
 
   virtual const char *natural_name () const = 0;
 
+  /* Digit separator of the language.  */
+
+  virtual const char *get_digit_separator () const
+  {
+    return " ";
+  }
+
   /* Return a vector of file extensions for this language.  The extension
      must include the ".", like ".c".  If this language doesn't need to
      provide any filename extensions, this may be an empty vector (which is