RISC-V: add Smrnmi 1.0 instruction support

RISC-V: add Smrnmi 1.0 instruction support

Add instruction `mnret' support

Ref:
https://github.com/riscv/riscv-isa-manual/blob/bb8b9127f81965eeff2d150c211d1c89376591c4/src/rnmi.adoc
https://github.com/riscv/riscv-opcodes/blob/946eb673874b3a0f2474d1424dc28bc7ee53c306/extensions/rv_smrnmi

bfd/ChangeLog:
    * elfxx-riscv.c: Add new Smrnmi instruction class handling

gas/ChangeLog:
    * testsuite/gas/riscv/smrnmi.s: New test for mnret
    * testsuite/gas/riscv/rmrnmi.d: Likewise

include/ChangeLog:
    * opcode/ricsv-opc.h: Add MATCH_MNRET, MASK_MNRET
    * opcode/riscv.h: Add new instruction class

opcodes/ChangeLog:
    * riscv-opc.c: Add `mnret' instruction

Signed-off-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>