scons: Make MOESI_CMP_directory the default ARM ruby protocol

scons: Make MOESI_CMP_directory the default ARM ruby protocol

Previously ARM binaries were by default compiled with the MI_example
protocol. The MI_example protocol cannot properly support load/store
exclusive instructions and therefore it cannot be used to simulate
multicore ARM systems. This change changes to MOESI_CMP_directory as
the default ruby protocol for ARM systems.

Change-Id: I942d950ba466aea9a75f3d8764f9f3eddd0c3baa
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2906
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>