RISC-V: Add support for Zfbfmin extension

This implements the Zfbfmin extension, as of version 1.0.

View detailed information in:
<https://github.com/riscv/riscv-isa-manual/blob/main/src/bfloat16.adoc#zfbfmin---scalar-bf16-converts>

1 The Zfbfmin extension depend on 'F', and the FLH, FSH, FMV.X.H, and
  FMV.H.X instructions as defined in the Zfh extension.

2 The Zfhmin extension includes the following instructions from the Zfh
  extension: FLH, FSH, FMV.X.H, FMV.H.X... View detailed information in:
  <https://github.com/riscv/riscv-isa-manual/blob/main/src/zfh.adoc>

3 Zfhmin extension depend on 'F'.

4 Simply put, just make Zfbfmin dependent on Zfhmin.

Perhaps in the future, we could propose making the FLH, FSH, FMV.X.H, and
FMV.H.X instructions an independent extension to achieve precise dependency
relationships for the Zfbfmin.

5 For relevant information in gcc, please refer to:
  <https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=35224ead63732a3550ba4b1332c06e9dc7999c31>

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_multi_subset_supports): Handle Zfbfmin.
	(riscv_multi_subset_supports_ext): Ditto.

gas/ChangeLog:

	* NEWS: Updated.
	* testsuite/gas/riscv/march-help.l: Ditto.
	* testsuite/gas/riscv/zfbfmin.d: New test.
	* testsuite/gas/riscv/zfbfmin.s: New test.

include/ChangeLog:

	* opcode/riscv-opc.h (MATCH_FCVT_BF16_S): Define.
	(MASK_FCVT_BF16_S): Ditto.
	(MATCH_FCVT_S_BF16): Ditto.
	(MASK_FCVT_S_BF16): Ditto.
	(DECLARE_INSN): New declarations for Zfbfmin.
	* opcode/riscv.h (enum riscv_insn_class): Add INSN_CLASS_ZFBFMIN.

opcodes/ChangeLog:

	* riscv-opc.c: Add Zfbfmin instructions.

include/opcode/riscv.h

@@ -449,6 +449,7 @@ enum riscv_insn_class
   INSN_CLASS_ZFHMIN_INX,
   INSN_CLASS_ZFHMIN_AND_D_INX,
   INSN_CLASS_ZFHMIN_AND_Q_INX,
+  INSN_CLASS_ZFBFMIN,
   INSN_CLASS_ZFA,
   INSN_CLASS_D_AND_ZFA,
   INSN_CLASS_Q_AND_ZFA,