Add a hook to allow extensions to veto xret. This will be used by CHERI extension. Note that illegal exception on mode check failure takes precedence over CHERI to allow for virtualisation.

    Rename var to make it clear it is physical and not virtual.

    There are a lot of variables in riscv_mem.sail and riscv_platform.sail named addr and it's not always clear if that is physical or virtual address. These changes rename those variables to paddr to reduce ambiguity.

Run Sail with -dno_cast even when it comes from opam package

Expand getopt_long option string to multiple lines

This will make future diffs easier to read and merge, since adding and
removing options will show up as entire lines that change instead of a
few characters in the middle of the line.

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.