RISC-V: Support Zicond extension

RISC-V: Support Zicond extension

    This implements the Zicond (conditional integer operations) extension,
    as of version 1.0-rc2.

    The Zicond extension acts as a building block for branchless sequences
    including conditional-arithmetic, conditional-logic and
    conditional-select/move.
    The following instructions constitute Zicond:
      - czero.eqz rd, rs1, rs2  =>  rd = (rs2 == 0) ? 0 : rs1
      - czero.nez rd, rs1, rs2  =>  rd = (rs2 != 0) ? 0 : rs1

    See
      https://github.com/riscv/riscv-zicond/releases/download/v1.0-rc2/riscv-zicond-v1.0-rc2.pdf
    for the proposed specification and usage details.

    bfd/ChangeLog:

            * elfxx-riscv.c (riscv_multi_subset_supports): Recognize
            INSN_CLASS_ZICOND.
            (riscv_multi_subset_supports_ext): Recognize INSN_CLASS_ZICOND.

    gas/ChangeLog:

            * testsuite/gas/riscv/zicond.d: New test.
            * testsuite/gas/riscv/zicond.s: New test.

    include/ChangeLog:

            * opcode/riscv-opc.h (MATCH_CZERO_EQZ): Define.
            (MASK_CZERO_EQZ): Define.
            (MATCH_CZERO_NEZ): Define,
            (MASK_CZERO_NEZ): Define.
            (DECLARE_INSN): Add czero.eqz and czero.nez.
            * opcode/riscv.h (enum riscv_insn_class): Add
            INSN_CLASS_ZICOND.

    opcodes/ChangeLog:

            * riscv-opc.c: Add czero.eqz and czero.nez.

    Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>