Default to arm64 cipd packages on arm macs

Enough works in that config that it seems like a better default by now.
Checking out, building, and uploading patches works (but uploading still
requires `--bypass-hooks` for now until crbug.com/1205263 is fixed).

(In case something important I'm not aware of breaks, we can revert this.)

Bug: 1103236,1205263
Change-Id: Id432ffd4ab19b15c019614953a2cefa787b46225
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2896468
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Nico Weber <thakis@chromium.org>

cipd

@@ -30,11 +30,6 @@ case "${UNAME}" in
     ;;
   darwin)
     OS=mac
-    # use amd64 binaries even on arm64 for now, see crbug.com/1205263
-    # Once we have arm binaries of everything, this will change to "arm64"
-    # Run `echo "mac-arm64" > .cipd_client_platform` to force arm64 binaries
-    # before everything is ready, for testing.
-    ARCH=amd64
     ;;
   *)
     >&2 echo "CIPD not supported on ${UNAME}"