Update GEM5 to master to fix x86 boot via 3feeb994ae613fd6b3734c1a991285b2ecbd1946

See also: http://gem5-users.gem5.narkive.com/CQh9J6fU/fail-to-run-fs-mode-with-linux-kernel-v4-8-13-and-ubuntu-image-16-04-1

Start sketching how to tweak system parameters, number of CPUs works, caches don't.

build-gem5

@@ -1,6 +1,6 @@
 #/usr/bin/env bash
 set -eu
-arch=arm
+arch=x86_64
 while getopts 'a:' OPT; do
   case "$OPT" in
     a)