RFR: 8186670: Implement _onSpinWait() intrinsic for AArch64 [v2]

[Evgeny Astigeevich]

On Tue, 21 Sep 2021 08:03:13 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Replace 'for' loops with macros
>
> src/hotspot/cpu/aarch64/aarch64.ad line 14374:
> 
>> 14372:         ShouldNotReachHere();
>> 14373:     }
>> 14374:     #undef EMIT_N_ASM_STRINGS
> 
> None of this is necessary. Printing "onspinwait" is enough.

This results no instructions implementing `onspinwait` in OptoAssembly output. Why do we want to hide the details?

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