RFR: 8359435: AArch64: add support for SB instruction to MacroAssembler::spin_wait

[Aleksey Shipilev]

On Wed, 25 Jun 2025 16:39:07 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

> I'm still waiting for a use for this thing.

The other project reports Evgeny linked in PR body look pretty convincing, as well as `ThreadOnSpinWait` microbenchmarks we have as well. This PR does not propose to _switch_ to `SB` for spin-waits, AFAICS. Just having `SB` as the spin-wait option does look fine to me.

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PR Comment: https://git.openjdk.org/jdk/pull/25801#issuecomment-3005475260