[PATCH v6] stdlib/strfrom: Add copysign to fix NAN issue on riscv (BZ #29501)

[Letu Ren]

> LGTM, thanks.  Really minor nits below that I did not catch before, there is no
> need to send a v7 (just fix it before install).

Thanks for your review!

> Double space after period.
>
> Double space before close comment.
>
> Double space after period and remove last final period.

I will pay attention to these style problems next time.

Regards
Letu Ren