RISC-V: Support Zacas extension.

https://github.com/riscvarchive/riscv-zacas/releases/tag/v1.0

The Zacas extension introduce compare-and-swap instructions to operate
on 32-bit, 64-bit and 128-bit (RV64 only) data values.

It introduces three new instructions:
  - amocas.w (32-bit CAS)
  - amocas.d (64-bit CAS)
  - amocas.q (128-bit CAS, RV64 only)

Like other AMOs in the A extension, Zacas instructions have '.aq',
'.rl' and '.aqrl' variations.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): 'A' implied by 'Zacas'.
	(riscv_supported_std_z_ext): Add 'Zacas' extension.
	(riscv_multi_subset_supports, riscv_multi_subset_supports_ext):
	Handle INSN_CLASS_ZACAS case.

gas/ChangeLog:

	* NEWS: Updated.
	* testsuite/gas/riscv/march-help.l: Updated.
	* testsuite/gas/riscv/zacas-32.d: New test (RV32).
        * testsuite/gas/riscv/zacas-fail-32.d: Likewise.
	* testsuite/gas/riscv/zacas-64.d: New test (RV64).
        * testsuite/gas/riscv/zacas-fail-64.d: Likewise.
	* testsuite/gas/riscv/zacas.s: New test source.
	* testsuite/gas/riscv/zacas-fail.s: Likewise.
	* testsuite/gas/riscv/zacas-fail-32.l: New file.
	* testsuite/gas/riscv/zacas-fail-64.l: Likewise.

include/ChangeLog:

	* include/opcode/riscv.h (INSN_CLASS_ZACAS): New definition.
	* include/opcode/riscv-opc.h (MATCH_AMOCAS_W, MASK_AMOCAS_W)
	(MATCH_AMOCAS_D, MASK_AMOCAS_D, MATCH_AMOCAS_Q, MASK_AMOCAS_Q):
	Likewise.
	(amocas_w, amocas_d, amocas_q): Declare instructions.

opcodes/ChangeLog:

	* riscv-opc.c (match_rs2_rd_even): New function.
	(amocas_w, amocas_d, amocas_q, amocas_w.aq)
	(amocas_d.aq, amocas_q.aq, amocas_w.rl, amocas_d.rl, amocas_q.rl)
	(amocas_w.aqrl, amocas_d.aqrl, amocas_q.aqrl): Add instructions.