riscv: Enable RVV selection in ifunc resolvers

[Yao Zihong]

This is a brand new series with a different granularity, but the same
question about whether hwcaps is consistently sufficient still applies.
For now I kept the hwcaps-based approach unchanged, and would like to
confirm if reviewers are comfortable with this direction.

https://patchwork.sourceware.org/project/glibc/patch/20250919005119.76910-4-zihong.plct@isrc.iscas.ac.cn/

Zihong