RISC-V: Add Zabha extension CAS instructions.

This patch update the cas instruction in Zabha extension [1],
when both Zabha and Zacas extension enabled.

[1] https://github.com/riscv/riscv-zabha/tags

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_multi_subset_supports): New extension case.

gas/ChangeLog:

	* testsuite/gas/riscv/zabha-32.d: New instructions.
	* testsuite/gas/riscv/zabha.d: Ditto.
	* testsuite/gas/riscv/zabha.s: Ditto.

include/ChangeLog:

	* opcode/riscv-opc.h (MATCH_AMOCAS_B): New opcodes.
	(MASK_AMOCAS_B): Ditto.
	(MATCH_AMOCAS_H): Ditto.
	(MASK_AMOCAS_H): Ditto.
	(DECLARE_INSN): New instructions.
	* opcode/riscv.h (enum riscv_insn_class): New class case.

opcodes/ChangeLog:

	* riscv-opc.c: New instructions.

include/opcode/riscv.h

@@ -493,6 +493,7 @@ enum riscv_insn_class
   INSN_CLASS_ZICBOZ,
   INSN_CLASS_ZABHA,
   INSN_CLASS_ZACAS,
+  INSN_CLASS_ZABHA_AND_ZACAS,
   INSN_CLASS_H,
   INSN_CLASS_XCVMAC,
   INSN_CLASS_XCVALU,