Add FENCE.TSO RV32I instruction

[elliotb-lowrisc]

Add optional RV32 instruction FENCE.TSO as fence_tso.

From the ISA:

The FENCE.TSO encoding was added as an optional extension to the original base FENCE instruction encoding. The base definition requires that implementations ignore any set bits and treat the FENCE as global, and so this is a backwards-compatible extension.

So it shouldn't break any existing implementations...

[PeterRugg]

Hmm, that's interesting. We should probably be testing all other values of the FM field in that case, although testrig is some way away from being able to find memory ordering violations: it's only really single core for a start! At least we should check the other encodings don't trap, which I could imagine being a common mistake...