RFR: 8346706: RISC-V: Add available registers to hs_err [v2]

[Robbin Ehn]

On Fri, 20 Dec 2024 13:58:35 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Thanks, I'll test it.
>
> It is written untested so it might need some tweaking. My suggestion also incorporated (silently) the recommendation to rename  $2RVV_FLAG to $2RVV_DEFINES. :)

I did a simpler version. Let me know :)
The tongue may have slipped :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/22845#discussion_r1894204369