Fix bisect-builds.py crash for non-official mac64 builds

Fix bisect-builds.py crash for non-official mac64 builds

There is no mac64 chromium distinction [1,2] and all builds are now
16 whole nibbles. This patch updates bisect-builds.py to use the
same codepath for mac64 and mac which has the nice effect of not
crashing when --archive=mac64 is used. Since ~m39, all chromium mac
builds have been 64bit.

TEST= tools/bisect-builds.py --archive=mac64 -g 330000 -b 340000

[1] http://commondatastorage.googleapis.com/chromium-browser-snapshots/index.html
[2] http://commondatastorage.googleapis.com/chromium-browser-snapshots/index.html?prefix=Mac/355973/

BUG=530321
Committed: https://crrev.com/db1d9ecccc0a59264de89067cb1ff354e42faeed
Cr-Commit-Position: refs/heads/master@{#356057}

[pdr., 2015-10-26 01:05:18]

pdr@chromium.org changed reviewers:
+ agable@chromium.org, stip@chromium.org

[pdr., 2015-10-26 01:08:39]

This file doesn't really have an owner (tools/OWNERS file says: *). Would either
of you be up for reviewing this with your most pythonic skills?

[pdr., 2015-10-26 03:01:06]

The CQ bit was checked by pdr@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-26 03:01:23]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1407263008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1407263008/1

[commit-bot: I haz the power, 2015-10-26 03:10:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-26 03:10:41]

Dry run: This issue passed the CQ dry run.

[ghost stip (do not use), 2015-10-26 06:10:05]

lgtm

[pdr., 2015-10-26 16:04:55]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2015-10-26 16:05:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1407263008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1407263008/1

[commit-bot: I haz the power, 2015-10-26 16:08:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-26 16:10:01]

Patchset 1 (id:??) landed as
https://crrev.com/db1d9ecccc0a59264de89067cb1ff354e42faeed
Cr-Commit-Position: refs/heads/master@{#356057}