lib: sbi_illegal_insn: Add emulation for fence.tso

While OpenC906 appears to properly decode `fence.tso` as a fence
instruction[1], the version of the C906 taped out in the Allwinner D1
does not, and raises illegal instruction.

Handle this errata by emulating `fence.tso` as `fence rw, rw`.

[1]: https://github.com/T-head-Semi/openc906/blob/30827e7f/C906_RTL_FACTORY/gen_rtl/idu/rtl/aq_idu_id_decd.v#L2097



Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Anup Patel <anup@brainfault.org>