config: Warn not fail for ARM systems configured with ruby

config: Warn not fail for ARM systems configured with ruby

Ruby for ARM systems is not fully supported but certain configurations
are expected to work. This change removes the more general fail
statement and warns or fails depending on the particular
configuration.

Change-Id: Ic24799aff966ba15858b93482e0f24a8672d9483
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2905
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>