[arm] Fix a double-register push operation.

[arm] Fix a double-register push operation.

It is not safe to access memory below sp, and this is also forbidden by
the ABI. When pushing, we must either use an atomic operation (such as
vstm+db_w) or move sp before writing the data.

This patch fixes one stack access, and also adds vpush and vpop helpers
to simplify similar code.

BUG=

Committed: https://crrev.com/f20d6461526eaf1ee1306e6e6048e06ae0f1688c
Cr-Commit-Position: refs/heads/master@{#31008}

[jbramley, 2015-09-29 08:43:10]

jacob.bramley@arm.com changed reviewers:
+ bmeurer@chromium.org, ulan@chromium.org

[Benedikt Meurer, 2015-09-29 14:45:06]

lgtm

[jbramley, 2015-09-29 14:45:40]

The CQ bit was checked by jacob.bramley@arm.com

[commit-bot: I haz the power, 2015-09-29 14:45:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1378563002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1378563002/1

[commit-bot: I haz the power, 2015-09-29 15:11:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-29 15:11:37]

Patchset 1 (id:??) landed as
https://crrev.com/f20d6461526eaf1ee1306e6e6048e06ae0f1688c
Cr-Commit-Position: refs/heads/master@{#31008}