RISC-V: Add .bfloat16 directive

RISC-V already support bfloat16 instruciton like Zfbfmin, Zvfbfmin and
Zvfbfwma, so I think it's reasonable to add .bfloat16 directive to
support bfloat16 data type.

And the code logic mostly support by common code already.
5 files changed