🐛 Make pipeline stall on all CSR accesses (#169)

[stmach]

Fixes bugs where changed CSR state is not visible to following instructions such as when changing the FP status CSR.
Fixes #169.

[davideschiavone]

Shall we merge @stmach ?

[stmach]

As in the current version of the core state-modifying CSRs have to be explicitly black-listed, this fix is kind of a "shotgun" approach to missing some registers - and hence loses performance.
A cleaner (and safer) approach would be to stall it by default (like this PR) but have a white-list of safe CSRs that don't need to flush the pipeline. We can merge this and think of opening a new issue for this as performance improvement.

Btw, the signal name csr_status_o that causes the pipeline to be flushed on CSR access is a bit misleading, consider refactoring.