NaCl cleanup: Use Bit('build_ARCH') instead of Bit('target_ARCH')

NaCl cleanup: Use Bit('build_ARCH') instead of Bit('target_ARCH')

This removes some uses of Bit('target_ARCH'), the definitions of which
have been removed from the NaCl side.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=2703
TEST=nacl_integration

Committed: https://crrev.com/0f96f55c8c4c36374494808451bfb335cbef49b0
Cr-Commit-Position: refs/heads/master@{#307044}

[Mark Seaborn, 2014-12-05 16:46:00]

mseaborn@chromium.org changed reviewers:
+ dschuff@chromium.org

[Derek Schuff, 2014-12-05 17:44:37]

lgtm

[Mark Seaborn, 2014-12-05 17:52:19]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2014-12-05 17:53:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/778173002/1

[commit-bot: I haz the power, 2014-12-05 18:00:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-05 18:02:45]

Patchset 1 (id:??) landed as
https://crrev.com/0f96f55c8c4c36374494808451bfb335cbef49b0
Cr-Commit-Position: refs/heads/master@{#307044}