This implements the m/senvcfg(h) CSRs. This CSR is used to enable/disable extensions and behaviours for lower privilege modes. Currently the only implemented bit is FIOM which affects how fences work.

It also affects how atomic memory accesses work in non-cacheable regions, but the model does not currently support PMAs so that can't easily be implemented.

This is a much clearer name because the option allows code to enable FIOM, it doesn't enable FIOM itself.