Intrinsic Support for BF16 Extension

[joshua-arch1]

The BF16 Extension has recently been proposed with three extra Instruction Set Extensions.
https://github.com/riscv/riscv-bfloat16

I'm wondering how we plan to address existing rvv intrinsics. Do we need to add new intrinsics tailored for bf16 datatype? If so, do we need to give a BF16 version for all the intrinsics with floating-point types? Maybe we can raise an issue for discussion.

[kito-cheng]

Could you create another issue on https://github.com/riscv-non-isa/rvv-intrinsic-doc/ for track RVV part?