RFR: 8291893: riscv: remove fence.i used in user space [v3]

[Yadong Wang]

On Mon, 8 Aug 2022 09:31:12 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

> Ok, I have run renaisance philosophers on the thead and sifive with jdk before and after the patch. seeing significant perf improvement on thead, 5-8% , with the patch. On hifive 0-3% perf improvement.
> 
> I can sponsor the commit if you want.

Thansk a lot. Please help with the sponsor.

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PR: https://git.openjdk.org/jdk/pull/9770