[aarch64-port-dev ] RFR: 8186670: Implement _onSpinWait() intrinsic for AArch64

[Dmitry Chuyko]

Please review implementation of Thread.onSpinWait() and some 
benchmarking around it. It would be great to get results from other HW.

study: http://cr.openjdk.java.net/~dchuyko/8186670/yield/spinwait.html
rfe: https://bugs.openjdk.java.net/browse/JDK-8186670
webrev: http://cr.openjdk.java.net/~dchuyko/8186670/webrev.00/

yield instruction was added to platform assembler. The intrinsic was 
enabled and emits that single instruction. Related test was enabled for 
the platform.

--
Dmitry Chuyko