Allow customizing CSR used by Zicsr extension

[nazar-pc]

Is your feature request related to a problem? Please describe.

Right now Zicsr tests use mscratch unconditionally, which is not a part of Zicsr specification and thus is not required for DUT to implement Zicsr correctly.

This prevents running Zicsr tests on otherwise perfectly valid RV32I_Zicsr_Zve32x target, for example.

Describe the solution you'd like

I think some way to configure which CSR tests should use would be nice.

Additional context

Initially mentioned in #1192 (comment)

[jordancarlin]

Beyond just DUTs that don't support mscratch, the proposal for the new boot sequence will put cores that support U-mode into U-mode when unpriv tests start, so mscratch won't be accessible.