[RFC] RISC-V: Support Zvabd extension instructions.

[Zhongyao Chen]

I accidentally duplicated some work. The related patch can be found at
the link below, just sharing here for everyone's awareness:

GCC patch, support for Zvabd proposal version 0.5:
https://gcc.gnu.org/pipermail/gcc-patches/2025-September/694552.html

binutils patch, , support for Zvabd proposal version 0.5:
https://sourceware.org/pipermail/binutils/2025-September/144102.html