Skip goma on unsupported platforms

Goma is not available on ppc/s390x platforms and causes
a crash when running gclient as it cannot find goma_ctl.py.

Change-Id: Icf08c6782d66f99eaf8f3c6ba42a170521afeb86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4545381
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

goma_ctl

@@ -17,4 +17,9 @@ MYPATH=$(dirname "${BASH_SOURCE[0]}")
 source "$MYPATH/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-PYTHONDONTWRITEBYTECODE=1 exec python3 "$MYPATH/.cipd_bin/goma_ctl.py" "$@"
+GOMA="$MYPATH/.cipd_bin/goma_ctl.py"
+if [[ -f "$GOMA" ]]; then
+    PYTHONDONTWRITEBYTECODE=1 exec python3 "$GOMA" "$@"
+else
+    echo "'$GOMA' was not found and may not be supported on this platform!"
+fi