package/gdb: fix build with glibc 2.20 (2)

Backport a second patch from gdb 7.8 for aarch64.

Fixes:
http://autobuild.buildroot.net/results/2fc/2fcf1a40a08baf881fdbccd070b1a1a6c89127e1/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Tested-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>