gdb/python: Export nibbles to python layer

This patch makes it possible to allow Value.format_string() to return
nibbles output.

When we set the parameter of nibbles to True, we can achieve the
displaying binary values in groups of every four bits.

Here's an example:
  (gdb) py print (gdb.Value (1230).format_string (format='t', nibbles=True))
  0100 1100 1110
  (gdb)

Note that the parameter nibbles is only useful if format='t' is also used.

This patch also includes update to the relevant testcase and
documentation.

Tested on x86_64 openSUSE Tumbleweed.