Revert "Remove tools/clang/scripts/download_objdump.py"

This reverts commit 29c801b5.

Reason for revert:
The official/chrome/chrome_android recipe (which does not show up
on internal code search) uses this, and that recipe also builds the 
M78 and M79 branches , so can't be migrated to use the new update.py
flag yet.

We'll have to keep this around until all builds are targeting M80 or
later.

Original change's description:
> Remove tools/clang/scripts/download_objdump.py
> 
> All uses have been replaced by calls to update.py --package=objdump.
> 
> Bug: 884608
> Change-Id: Icb820782786a256c0a3586e8a23f62d463689afb
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917517
> Auto-Submit: Hans Wennborg <hans@chromium.org>
> Commit-Queue: Reid Kleckner <rnk@chromium.org>
> Reviewed-by: Reid Kleckner <rnk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#715750}

TBR=hans@chromium.org,rnk@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 884608, 1025703
Change-Id: Ib048dfcbd91f4f5201a6b77525189e6185dc7e80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921977Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716151}

tools/clang/scripts/download_objdump.py

+#!/usr/bin/env python
+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import sys
+
+import update
+
+# TODO(hans): Remove this forwarding hack after all callers of this script have
+# been updated to call update.py instead.
+if __name__ == '__main__':
+  sys.argv = [sys.argv[0], '--package=objdump']
+  sys.exit(update.main())