Fix host_arch detection for AIX and one new warning as error

Fix host_arch detection for AIX and one new warning as error

The value returned on AIX for platform.machine() is not the
best value to map the architecture from.  Use platform.system
to determine if we are on AIX and if so set host_arch to
ppc64 as AIX 6.1 (the earliest supported) only provides a
64 bit kernel

AIX was reporting warning that offset may be used uninitialized

	modified:   build/detect_v8_host_arch.py
	modified:   build/standalone.gypi
	modified:   src/hydrogen-bce.cc

R=mbrandy@us.ibm.com, jkummerow@chromium.org

BUG=

Committed: https://crrev.com/2ec0f32abbf474fab6e618c0056765b483a66b2f
Cr-Commit-Position: refs/heads/master@{#27463}

[michael_dawson, 2015-03-25 20:52:54]

michael_dawson@ca.ibm.com changed reviewers:
+ jkummerow@chromium.org
- svenpanne@chromium.org

[michael_dawson, 2015-03-25 20:54:16]

Fixes to get AIX building again.

[Sven Panne, 2015-03-26 07:22:17]

svenpanne@chromium.org changed reviewers:
+ svenpanne@chromium.org

[Sven Panne, 2015-03-26 07:22:18]

Drive-by LGTM...

[Sven Panne, 2015-03-26 07:22:21]

The CQ bit was checked by svenpanne@chromium.org

[commit-bot: I haz the power, 2015-03-26 07:22:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1006583004/1

[commit-bot: I haz the power, 2015-03-26 08:06:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-26 08:07:05]

Patchset 1 (id:??) landed as
https://crrev.com/2ec0f32abbf474fab6e618c0056765b483a66b2f
Cr-Commit-Position: refs/heads/master@{#27463}