[PATCH v5 14/18] riscv: Add RVV strncat for multiarch and non-multiarch

[Zihong Yao]

Hi Peter,

For `memrchr` and `strnlen`, the generic implementations are on `__memrchr`
and `__strnlen`, with `memrchr`/`strnlen` just being aliases to them. So in
those two cases, it seemed more natural to make `__memrchr`/`__strnlen` themselves
the IFUNC symbols, and the redirected form makes that straightforward.

But `strncat` and `__memcmpeq` are a bit different. I mainly kept that form there
because I was following the x86_64 pattern. `strncpy` already uses the simpler
form.

I'm happy to rework them if you prefer to keep cases aligned.

Zihong