Mac ASan 64 builder - Add cores: to dimensions

Going to switch this to a larger bot and turn off Goma for the time being.

Change-Id: Ia09d67b6acdb03ba1228ea3a73bc41da959d1dba
Reviewed-on: https://chromium-review.googlesource.com/1112477Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569730}

infra/config/global/cr-buildbucket.cfg

@@ -1573,6 +1573,7 @@ buckets {
     builders {
       name: "Mac ASan 64 Builder"
       dimensions: "os:Mac-10.13"
+      dimensions: "cores:"  # Swapping between 8 and 24
       mixins: "memory-ci"
     }
     builders {