RISC-V: Add support for Zvfbfwma extension

RISC-V: Add support for Zvfbfwma extension

This implements the Zvfbfwma extension, as of version 1.0.
View detailed information in:
<https://github.com/riscv/riscv-isa-manual/blob/main/src/bfloat16.adoc#zvfbfwma---vector-bf16-widening-mul-add>

1 In spec: "Zvfbfwma requires the Zvfbfmin extension and the Zfbfmin extension."
  1.1 In Embedded    Processor: Zvfbfwma -> Zvfbfmin -> Zve32f
  1.2 In Application Processor: Zvfbfwma -> Zvfbfmin -> V
  1.3 In both scenarios, there are: Zvfbfwma -> Zfbfmin

2 Depending on different usage scenarios, the Zvfbfwma extension may
depend on 'V' or 'Zve32f'. This patch only implements dependencies in
scenario of Embedded Processor. This is consistent with the processing
strategy in Zvfbfmin. In scenario of Application Processor, it is
necessary to explicitly indicate the dependent 'V' extension.

For relevant information in gcc, please refer to:
<https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=38dd4e26e07c6be7cf4d169141ee4f3a03f3a09d>

bfd/ChangeLog:

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

gas/ChangeLog:

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

include/ChangeLog:

	* opcode/riscv-opc.h (MATCH_VFWMACCBF16_VF): Define.
	(MASK_VFWMACCBF16_VF): Ditto.
	(MATCH_VFWMACCBF16_VV): Ditto.
	(MASK_VFWMACCBF16_VV): Ditto.
	(DECLARE_INSN): New declarations for Zvfbfwma.
	* opcode/riscv.h (enum riscv_insn_class): Add
	INSN_CLASS_ZVFBFWMA

opcodes/ChangeLog:

	* riscv-opc.c: Add Zvfbfwma instructions.