RISC-V: Add support for the Zifencei extension

RISC-V: Add support for the Zifencei extension

fence.i has been split out of the base ISA as part of the ratification
process.  This patch adds a Zifencei argument, which disables the
fence.i instruction.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>