GN: Add nacl_arm_glibc toolchain

GN: Add nacl_arm_glibc toolchain

This adds a toolchain definition for NaCl's ARM glibc toolchain.
Nothing uses this yet.  Once this lands, changes in the native_client
repo will be required to start using it.

BUG=512901
R=dpranke@chromium.org, phosek@chromium.org

Committed: https://crrev.com/ab949d7831e67970d671352cf7815b4e52c93b9b
Cr-Commit-Position: refs/heads/master@{#355398}

[Dirk Pranke, 2015-10-21 20:10:13]

lgtm

[Roland McGrath, 2015-10-21 20:13:47]

The CQ bit was checked by mcgrathr@chromium.org

[commit-bot: I haz the power, 2015-10-21 20:14:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1411743004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1411743004/1

[commit-bot: I haz the power, 2015-10-21 21:06:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-21 21:07:35]

Patchset 1 (id:??) landed as
https://crrev.com/ab949d7831e67970d671352cf7815b4e52c93b9b
Cr-Commit-Position: refs/heads/master@{#355398}