Enable fake NUMA (performance optimization)

[stapelberg]

See https://forums.raspberrypi.com/viewtopic.php?t=378276 for context.

Currently, we build without NUMA:

/tmp/breakglass207482315 # uname -a
Linux scan2drive 6.6.51-v8 #1 SMP PREEMPT Wed Mar  1 20:57:29 UTC 2017 aarch64 GNU/Linux
/tmp/breakglass207482315 # gunzip -c /proc/config.gz  | grep -i numa
CONFIG_ARCH_SUPPORTS_NUMA_BALANCING=y
# CONFIG_NUMA is not set

I am not sure if enabling NUMA is sufficient for any performance wins alone (that might change in the next few weeks).