ggml-cpu : add RISC-V Zvfh impl for ggml_vec_mad_f16

[xctan]

This commit adds a RISC-V vector intrinsic implementation for ggml_vec_mad_f16 when the Zvfh extension is present.

[ggerganov]

Recently we made the following changes to deduplicate the scalar code:

#17314

Would be nice to make a pass on this code and make the ifdefs consistent across all functions. Probably a follow-up PR if you are interested.

[luhenry]

This seem to have duplicated work done at #17318 as well.