Add the RISC-V bits to gas/NEWS

[Nelson Chu]

OK, these should mention all the past updates, including the 2.39, so committed.

Thanks
Nelson

On Thu, Aug 25, 2022 at 4:20 AM Palmer Dabbelt <palmer@rivosinc.com> wrote:
>
> This is somewhat embarassing, but looks like we've neglected to add NEWS
> entries since the one announcing our port.  I've got through the history
> and tried to pull everything out, I think I got all the big stuff like
> new extensions.
>
>