Implemented l[w|h|b]arx and st[w|h|b]cx instructions which are
needed to perform atomic exchange. Also added synchronization
primitives similar to arm to simulate those instructions.

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, binji@chromium.org, aseemgarg@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2754263004
Cr-Commit-Position: refs/heads/master@{#44257}