Add some effects to execute function declarationand remove val specs for calling functions to allow effects to be inferred.

Also fix "clean" target in Holmakefile

Make a custom exception code available for extensions, and remove the E_CHERI code.  Enable extensions for PTE checks and PTW errors, and propagate those into exception codes.

Add a new pc access function to get the architectural PC: on CHERI this is not what is in the PC register because the architectural PC is the offset of PCC and the PC register stores the absolute PC (at present -- may review this decision in future). This allows fix for AUIPC and RVFI reported PC.

Move updated 0.11 lem files from Shaked's commit into their own directory

Remove the 0.7.1 lem directory that performed a similar purpose during the prompt monad changes

Re-add model changes from Shaked's commit with a feature flag

Move this commit to new_barriers branch until we can update C/SMT/etc
and make a new release of Sail for backwards compatability with the
opam package

This reverts commit 273ec8b0.

Disable mmio devices (clint and htif interfaces) when using RVFI to prevent divergence when using TestRIG.

Use reserved bits in PTEs for vmem extensions on RV64, as allowed by the spec.  This is not possible for RV32, so pass zeros there.