RISC-V: Add support for Zimop extension

This implements the Zimop (May-Be-Operations) extension, as of version 1.0.

View detailed information in:
<https://github.com/riscv/riscv-isa-manual/blob/main/src/zimop.adoc>

bfd/ChangeLog:

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

gas/ChangeLog:

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

include/ChangeLog:

	* opcode/riscv-opc.h (DECLARE_INSN): New declarations for Zimop.
	(MATCH_MOP_R_0, MATCH_MOP_R_1, MATCH_MOP_R_2, MATCH_MOP_R_3,
	MATCH_MOP_R_4, MATCH_MOP_R_5, MATCH_MOP_R_6, MATCH_MOP_R_7,
	MATCH_MOP_R_8, MATCH_MOP_R_9, MATCH_MOP_R_10, MATCH_MOP_R_11,
	MATCH_MOP_R_12, MATCH_MOP_R_13, MATCH_MOP_R_14, MATCH_MOP_R_15,
	MATCH_MOP_R_16, MATCH_MOP_R_17, MATCH_MOP_R_18, MATCH_MOP_R_19,
	MATCH_MOP_R_20, MATCH_MOP_R_21, MATCH_MOP_R_22, MATCH_MOP_R_23,
	MATCH_MOP_R_24, MATCH_MOP_R_25, MATCH_MOP_R_26, MATCH_MOP_R_27,
	MATCH_MOP_R_28, MATCH_MOP_R_29, MATCH_MOP_R_30, MATCH_MOP_R_31,
	MATCH_MOP_RR_0, MATCH_MOP_RR_1, MATCH_MOP_RR_2, MATCH_MOP_RR_3,
	MATCH_MOP_RR_4, MATCH_MOP_RR_5, MATCH_MOP_RR_6, MATCH_MOP_RR_7): Define.
	(MASK_MOP_R_0, MASK_MOP_R_1, MASK_MOP_R_2, MASK_MOP_R_3, MASK_MOP_R_4,
	MASK_MOP_R_5, MASK_MOP_R_6, MASK_MOP_R_7, MASK_MOP_R_8, MASK_MOP_R_9,
	MASK_MOP_R_10, MASK_MOP_R_11, MASK_MOP_R_12, MASK_MOP_R_13,
	MASK_MOP_R_14, MASK_MOP_R_15, MASK_MOP_R_16, MASK_MOP_R_17,
	MASK_MOP_R_18, MASK_MOP_R_19, MASK_MOP_R_20, MASK_MOP_R_21,
	MASK_MOP_R_22, MASK_MOP_R_23, MASK_MOP_R_24, MASK_MOP_R_25,
	MASK_MOP_R_26, MASK_MOP_R_27, MASK_MOP_R_28, MASK_MOP_R_29,
	MASK_MOP_R_30, MASK_MOP_R_31, MASK_MOP_RR_0, MASK_MOP_RR_1,
	MASK_MOP_RR_2, MASK_MOP_RR_3, MASK_MOP_RR_4, MASK_MOP_RR_5,
	MASK_MOP_RR_6, MASK_MOP_RR_7): Ditto.
	* opcode/riscv.h (enum riscv_insn_class): Add INSN_CLASS_ZIMOP.

opcodes/ChangeLog:

	* riscv-opc.c: Add Zimop instructions.