RFR (11): Revert to emit locked cmpxchg only on MP

[Roman Kennke]

Last backports introduced a minor diff vs. upstream that we can and
should revert:

http://cr.openjdk.java.net/~rkennke/jdk11-revert-lock-cas/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman