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=

Review URL: https://codereview.chromium.org/1378563002

Cr-Commit-Position: refs/heads/master@{#31008}