misc: Update Garnet_standalone build opts

misc: Update Garnet_standalone build opts

Previously, this used the ALPHA ISA which has been removed.

According to the documentation on the website
(https://www.gem5.org/documentation/general_docs/ruby/
garnet_synthetic_traffic/), using null ISA with no CPUs should be safe.

Change-Id: I7fc55df7217887d21f4832f33b8cbb2de3498253
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29087
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>