arch-arm: Fixed issue building ARM_MESI_Three_Level

Change-Id: I1ef200cd282e189d142a5902b6ddbd33119c4173
Jira: https://gem5.atlassian.net/browse/GEM5-594
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29352
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
diff --git a/build_opts/ARM_MESI_Three_Level b/build_opts/ARM_MESI_Three_Level
index 1836fb0..29a429c 100644
--- a/build_opts/ARM_MESI_Three_Level
+++ b/build_opts/ARM_MESI_Three_Level
@@ -2,5 +2,5 @@
 # All rights reserved.
 
 TARGET_ISA = 'arm'
-CPU_MODELS = 'TimingSimpleCPU, O3CPU'
+CPU_MODELS = 'TimingSimpleCPU,O3CPU'
 PROTOCOL = 'MESI_Three_Level'