[telemetry] Fix MSR power monitor on Windows bots.

[telemetry] Fix MSR power monitor on Windows bots.

I VNCed into a bot and learned that the Buildbot scripts munge the environment
variables, so that PROGRAMFILES(X86) doesn't exist. Using platform.machine()
is a more robust, cross-platform way of checking if the OS is 64-bit.

The Windows XP bots still worked, because their OSes are all 32-bit.

BUG=336556
TEST=None.

Committed: https://crrev.com/3bbeaa6aa2cc762ceac9c886660af7b0db59c346
Cr-Commit-Position: refs/heads/master@{#296564}

[dtu, 2014-09-24 21:57:35]

dtu@chromium.org changed reviewers:
+ tonyg@chromium.org

[tonyg, 2014-09-24 22:16:39]

The CQ bit was checked by tonyg@chromium.org

[tonyg, 2014-09-24 22:16:39]

lgtm

[commit-bot: I haz the power, 2014-09-24 22:18:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/603763002/1

[commit-bot: I haz the power, 2014-09-24 23:37:28]

Committed patchset #1 (id:1) as 1b29260893f93e2027aab3ea72a1374b327f72bf

[commit-bot: I haz the power, 2014-09-24 23:47:05]

Patchset 1 (id:??) landed as
https://crrev.com/3bbeaa6aa2cc762ceac9c886660af7b0db59c346
Cr-Commit-Position: refs/heads/master@{#296564}