package/gdb: fix gdb python support with python 3.8

package/gdb: fix gdb python support with python 3.8

CPython 3.8 has added a new Syntax Warning that print
a waring at runtime while unsing gdb python support.

$ gdb -ex "python import os"
/usr/share/gdb/python/gdb/command/prompt.py:48: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if self.value is not '':
/usr/share/gdb/python/gdb/command/prompt.py:60: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if self.value is not '':

Backport an upstream patch for this:
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=b6484282f85bf7f11451b2441599c241d302ad9d

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>