RFR: JDK-8199735: Mark word updates need to use Access API

[Roman Kennke]

Currently, the mark word is accessed directly in oopDesc::mark()
set_mark() and a bunch of other accessors. Those need to use the Access
API instead because GC might want to employ barriers on those accesses,
pretty much like every other field in oopDesc.

Notice that this is *not* about accessing the bits and fields inside the
markOop, but about accessing the header itself.

http://cr.openjdk.java.net/~rkennke/JDK-8199735/webrev.01/

Testing: build fastdebug/release with and without PCH, passed tier1
fastdebug/release.

Can I please get reviews?

Thanks, Roman

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