User mode support

[mkurc-ant]

This PR adds User mode support to VeeR EL2 core. This implies also:

• enabling U bit in misa
• adding new fields to mstatus CSR
• adding CSR access control
• adding new exception code for ECALL.U instruction.
• preventing privilege instructions from running in user mode
• adding user mode accessible shadow CSRs for performance counters
• adding mcounteren CSR that controls access to counter shadow CSRs
• updating PMP behavior to account for user mode

Along with user mode support there's also extended PMP (Smepmp) support added.

Both user mode and Smepmp are disabled by default. They need to be enabled via veer.config script. To enable user mode append:

-set=user_mode=1

to its invocation. And to enable Smepmp append:

-set=smepmp=1

With the PR there are new tests added that exercise new features.

[github-actions[bot]]

Links to coverage and verification reports for this PR (#187) are available at https://chipsalliance.github.io/Cores-VeeR-EL2/

[github-actions[bot]]

Links to coverage and verification reports for this PR (#187) are available at https://chipsalliance.github.io/Cores-VeeR-EL2/

[github-actions[bot]]

Links to coverage and verification reports for this PR (#187) are available at https://chipsalliance.github.io/Cores-VeeR-EL2/

[kgugala]

LGTM