pw_env_setup: Use arm-gcc 9 in compatibility.json

Use arm-gcc 9 in compatibility.json so we ensure it continues to build
upstream Pigweed, since some downstream projects don't want to switch to
arm-gcc 10 anytime soon.

Change-Id: If9aeba05ccb1d45681394b01efca7cde81c6458b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/43982
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_env_setup/py/pw_env_setup/cipd_setup/compatibility.json b/pw_env_setup/py/pw_env_setup/cipd_setup/compatibility.json
index 9132342..c16881f 100644
--- a/pw_env_setup/py/pw_env_setup/cipd_setup/compatibility.json
+++ b/pw_env_setup/py/pw_env_setup/cipd_setup/compatibility.json
@@ -4,5 +4,11 @@
     "tags": [
       "version:3.7.7.chromium.10"
     ]
+  },
+  {
+    "path": "pigweed/third_party/gcc-arm-none-eabi/${os}-${arch=amd64}",
+    "tags": [
+      "version:9-2020-q2-update"
+    ]
   }
 ]