Fix 32-bit x86 Linux build for GN uploads.

Fix 32-bit x86 Linux build for GN uploads.

The bots still use 'cpu_arch' to set the architecture, and the build
file was keying off of build_cpu_arch to determine whether or not
to enable NaCl, so things were failing. Updating the the code to
'current_cpu' fixes the problem correctly.

TBR=brettw@chromium.org

Committed: https://crrev.com/1c35162ea310ab09b3bb72fe1bfe95232db5d1a6
Cr-Commit-Position: refs/heads/master@{#318585}

[Dirk Pranke, 2015-02-28 01:15:02]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-28 01:16:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/964003002/1

[commit-bot: I haz the power, 2015-02-28 02:55:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-28 02:56:49]

Patchset 1 (id:??) landed as
https://crrev.com/1c35162ea310ab09b3bb72fe1bfe95232db5d1a6
Cr-Commit-Position: refs/heads/master@{#318585}