RISC-V: Add support for Zcmop extension

This implements the Zcmop (Compressed Zimop) extension, as of version 1.0.

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

The Zcmop extension requires the Zca extension.

bfd/ChangeLog:

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

gas/ChangeLog:

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

include/ChangeLog:

	* opcode/riscv-opc.h (DECLARE_INSN): New declarations for Zcmop.
	(MATCH_C_MOP_1, MATCH_C_MOP_3, MATCH_C_MOP_5, MATCH_C_MOP_7,
	MATCH_C_MOP_9, MATCH_C_MOP_11, MATCH_C_MOP_13, MATCH_C_MOP_15): Define.
	(MASK_C_MOP_1, MASK_C_MOP_3, MASK_C_MOP_5, MASK_C_MOP_7,
	MASK_C_MOP_9, MASK_C_MOP_11, MASK_C_MOP_13, MASK_C_MOP_15): Ditto.
	* opcode/riscv.h (enum riscv_insn_class): Add INSN_CLASS_ZCMOP.

opcodes/ChangeLog:

	* riscv-opc.c: Add Zcmop instructions.