1 At point <https://github.com/riscv/riscv-bfloat16>,
  BF16 has already been completed "post public review".

2 LLVM has also added support for RISCV BF16 in
  <https://reviews.llvm.org/D151313> and
  <https://reviews.llvm.org/D150929>.

3 According to the discussion <https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/367

>,
  this use __bf16 and use DF16b in riscv_mangle_type like x86.

Below test are passed for this patch
    * The riscv fully regression test.

gcc/ChangeLog:

	* config/riscv/iterators.md: New mode iterator HFBF.
	* config/riscv/riscv-builtins.cc (riscv_init_builtin_types):
	Initialize data type _Bfloat16.
	* config/riscv/riscv-modes.def (FLOAT_MODE): New.
	(ADJUST_FLOAT_FORMAT): New.
	* config/riscv/riscv.cc (riscv_mangle_type): Support for BFmode.
	(riscv_scalar_mode_supported_p): Ditto.
	(riscv_libgcc_floating_mode_supported_p): Ditto.
	(riscv_init_libfuncs): Set the conversion method for BFmode and
	HFmode.
	(riscv_block_arith_comp_libfuncs_for_mode): Set the arithmetic
	and comparison libfuncs for the mode.
	* config/riscv/riscv.md (mode" ): Add BF.
	(movhf): Support for BFmode.
	(mov<mode>): Ditto.
	(*movhf_softfloat): Ditto.
	(*mov<mode>_softfloat): Ditto.

libgcc/ChangeLog:

	* config/riscv/sfp-machine.h (_FP_NANFRAC_B): New.
	(_FP_NANSIGN_B): Ditto.
	* config/riscv/t-softfp32: Add support for BF16 libfuncs.
	* config/riscv/t-softfp64: Ditto.
	* soft-fp/floatsibf.c: For si -> bf16.
	* soft-fp/floatunsibf.c: For unsi -> bf16.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/bf16_arithmetic.c: New test.
	* gcc.target/riscv/bf16_call.c: New test.
	* gcc.target/riscv/bf16_comparison.c: New test.
	* gcc.target/riscv/bf16_float_libcall_convert.c: New test.
	* gcc.target/riscv/bf16_integer_libcall_convert.c: New test.

Co-authored-by: Jin Ma <jinma@linux.alibaba.com>