[PING^3][v2] RISC-V: Add SpacemiT vendor extensions xsmtvdot

[Mark Zhuang]

Hi Michal,

Thank you for pointing out the HTML formatting issue. I apologize for
the confusion it may have caused.

My original patch submission was sent using git send-email,
but the subsequent PING replies were sent directly from my email client,
which inadvertently mixed in HTML formatting. When I reviewed the thread
at https://sourceware.org/pipermail/binutils/2026-April/148909.html, I
didn't notice the rendering issues, so I wasn't aware of the problem
until you mentioned it.

I'm very sorry for any inconvenience the HTML formatting may have caused.
I've now attached the clean patch below.

Best regards,
Mark Zhuang