RFR: 8186670: Implement _onSpinWait() intrinsic for AArch64 [v15]

[Evgeny Astigeevich]

> This PR is a follow-up on the discussion [“RFC: AArch64: Implementing spin pauses with ISB”](https://mail.openjdk.java.net/pipermail/hotspot-dev/2021-August/054033.html).
> 
> It adds DIAGNOSTIC options `OnSpinWaitInst=inst`, where `inst` can be:
> 
> - `none`: no implementation for spin pauses. This is the default value.
> - `nop`: use `nop` instruction for spin pauses.
> - `isb`: use `isb` instruction for spin pauses.
> - `yield`: use `yield` instruction for spin pauses.
> 
> And  `OnSpinWaitInstCount=count`, where `count` specifies a number of `OnSpinWaitInst` and can be in `1..99` range. It is an error to use `OnSpinWaitInstCount` when `OnSpinWaitInst` is `none`.
> 
> The code for the `Thread.onSpinWait` intrinsic is generated based on the values of `OnSpinWaitInst` and `OnSpinWaitInstCount`.
> 
> Testing:
> 
> - `make test TEST="gtest"`: Passed
> - `make run-test TEST="tier1"`: Passed
> - `make run-test TEST="tier2"`: Passed
> - `make run-test TEST=hotspot/jtreg/compiler/onSpinWait`: Passed
> 
> CSR: https://bugs.openjdk.java.net/browse/JDK-8274564

Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:

  8275728: Add simple Producer/Consumer microbenchmark for Thread.onSpinWait

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5562/files
  - new: https://git.openjdk.java.net/jdk/pull/5562/files/a06b4821..0d6fc3f0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5562&range=14
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5562&range=13-14

  Stats: 204 lines in 1 file changed: 204 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5562.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5562/head:pull/5562

PR: https://git.openjdk.java.net/jdk/pull/5562