RFR: JDK-8205523: Explicit barriers for interpreter

[Roman Kennke]

Hi all,

A number of operations cannot reasonably make use of the Access API but
require explicit read- and write-barriers for GCs like Shenandoah that
need to ensure to-space consistency. Examples are monitor-enter/-exit
and some intrinsics.

The change adds APIs to BarrierSetAssembler (x86 and aarch64) to support
these kinds of explicit barriers, and the necessary calls in relevant
places. The default implementation does nothing. These barriers have
been found and tested over several years in Shenandoah.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8205523
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8205523/webrev.00/

Testing: hotspot/tier1, will submit into Mach5 after reviews.

Can I please get reviews?

Thanks, Roman