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 <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29087
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5