RISC-V: Control fence.i and csr instructions by zifencei and zicsr.

bfd/
    * elfxx-riscv.c (riscv_ext_dont_care_version): New function.  Return
    TRUE if we don't care the versions of the extensions.  These extensions
    are added to the subset list for special purposes, with the explicit
    versions or the RISCV_UNKNOWN_VERSION versions.
    (riscv_parse_add_subset): If we do care the versions of the extension,
    and the versions are unknown, then report errors for the non-implicit
    extensions, and return directly for the implicit one.
    (riscv_arch_str1): Do not output i extension after e, and the extensions
    which versions are unknown.
gas/
    * config/tc-riscv.c (riscv_multi_subset_supports): Handle INSN_CLASS_ZICSR
    and INSN_CLASS_ZIFENCEI.
    * testsuite/gas/riscv/march-imply-i.s: New testcase.
    * testsuite/gas/riscv/march-imply-i2p0-01.d: New testcase.  The version
    of i is less than 2.1, and zi* are supported in the chosen spec, so
    enable the fence.i and csr instructions, also output the implicit zi* to
    the arch string.
    * testsuite/gas/riscv/march-imply-i2p0-02.d: Likewise, but the zi* are
    not supported in the spec 2.2.  Enable the related instructions since
    i's version is less than 2.1, but do not output them.
    * testsuite/gas/riscv/march-imply-i2p1-01.d: New testcase.  The version
    of i is 2.1, so don't add it's implicit zi*, and disable the related
    instructions.
    * testsuite/gas/riscv/march-imply-i2p1-01.l: Likewise.
    * testsuite/gas/riscv/march-imply-i2p1-02.d: Likewise, and set the zi*
    explicitly, so enable the related instructions.
    * testsuite/gas/riscv/march-imply-i2p0.d: Removed.
    * testsuite/gas/riscv/march-imply-i2p1.d: Removed.
include/
    * opcode/riscv.h: Add INSN_CLASS_ZICSR and INSN_CLASS_ZIFENCEI.
opcodes/
    * riscv-opc.c (riscv_opcodes): Control fence.i and csr instructions by
    zifencei and zicsr.

include/opcode/riscv.h

@@ -306,9 +306,11 @@ enum riscv_insn_class
    INSN_CLASS_M,
    INSN_CLASS_F,
    INSN_CLASS_D,
-   INSN_CLASS_D_AND_C,
-   INSN_CLASS_F_AND_C,
    INSN_CLASS_Q,
+   INSN_CLASS_F_AND_C,
+   INSN_CLASS_D_AND_C,
+   INSN_CLASS_ZICSR,
+   INSN_CLASS_ZIFENCEI,
   };
 
 /* This structure holds information for a particular instruction.  */