[RFC] RISC-V: Support risc-v bfloat16 This patch support bfloat16 in riscv like x86_64 and arm.

[Jin Ma via Gcc-patches]

hi, 

Are there any new developments about Zfb? Are there any plans to implement
the Zvfbfmin and Zvfbfwma expansion? I see that Zfb is being reviewed in
llvm, maybe we should do the same on gcc.

Ref: https://reviews.llvm.org/D151313
     https://reviews.llvm.org/D150929