Fix worker race condition in gemmlowp. Bug: 29993722 - Merge upstream change 67949d5f0f0e00d44709570d87f5ffa43b81e4f1 - Fix race condition by adding explicit memory barriers Change-Id: Ie3e72ac3307e6b205d9f473aac3eb19ea43866f8 (cherry picked from commit d2f96f0c9363004d5bd8d042dbc70c1519875d33)