RFR: 8184181: Use oopDesc::cas_set_mark() instead of raw CAS when accessing oop header

[Roman Kennke]

BiasedLocking and ObjectSynchronizer both use raw Atomic::cmpxchg_ptr()
calls to CAS the header word of an oop. It's cleaner to use
oopDesc::cas_set_mark() instead. Besides this being a cleanup, in the
future (Shenandoah) we will also require GC barriers to do the right
thing there.

Webrev:
http://cr.openjdk.java.net/~rkennke/8184181/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/8184181/webrev.00/>

Bug:
https://bugs.openjdk.java.net/browse/JDK-8184181

Ok?

Tested by running hotspot_runtime tests.

Roman