[riscv-port] RFR: 8277440: riscv: Move UseVExt from product to experimental [v2]

[Yanhong Zhu]

On Fri, 19 Nov 2021 09:09:05 GMT, Yadong Wang <yadongwang at openjdk.org> wrote:

> It's better to write like this:
> 
> ```
>   product(bool, UseRTMForStackLocks, false, EXPERIMENTAL,                   \
>           "Enable RTM lock eliding for stack locks in compiled code")       \
> ```

Done.

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PR: https://git.openjdk.java.net/riscv-port/pull/11