Add arch flag and env variable for overriding on mac

R=sokcevic@google.com

Change-Id: I2c07c2157a404b0eb29da8b46b22990a470576c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4005186
Commit-Queue: Theodore Siu <theosiu@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>

cipd

@@ -30,6 +30,10 @@ case "${UNAME}" in
     ;;
   darwin)
     OS=mac
+    # Allow mac users to override easily override arch detection
+    if [ ! -z "${ARCH_MAC_OVERRIDE}" ]; then
+      ARCH="${ARCH_MAC_OVERRIDE}"
+    fi
     ;;
   *)
     >&2 echo "CIPD not supported on ${UNAME}"