[RISCV] Implement MC layer support for the fence.tso instruction
The instruction makes use of a previously ignored field in the fence
instruction. It is introduced in the version 2.3 draft of the RISC-V
specification after much work by the Memory Model Task Group.
As clarified here https://github.com/riscv/riscv-isa-manual/issues/186,
the fence.tso assembler mnemonic does not have operands.