[RISCV][BF16] Enable __bf16 for riscv targets

The RISC-V psABI recently added __bf16 in https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/367.
Now we can enable this new type in clang.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D150929