util-docker: Add gdb to gcn-gpu Dockerfile

util-docker: Add gdb to gcn-gpu Dockerfile

gdb is generally useful.

Change-Id: Ic73822a5f61914cafba0699949dccabc81c03d2a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39096
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Kyle Roarty <kyleroarty1716@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>