Allow extensions to provide their own exception codes/names

Matches recent changes to Sail

The workaround from previous commit did not work for sail-cheri-riscv (I think it is a problem with topological sorting that is highly dependent on the order in which functions are defined and used). Unfortunately this means we no longer have CSR names in disassembly, only numbers.

Re-instate the catch-all csr_name_map clause and workaround the Sail bug that prevented this from compiling previously.
This will stop crashes on invalid CSRs in TestRIG. I have a reduced test case for the Sail bug and will investigate.

Fix parsing long options in the C emulator for RVFI-DII

idx, passed as longindex to getopt_long, is updated every time a long
option is parsed to be the index of that option in the options array,
not argv. We should instead use optind as in the non-RVFI-DII case, and
we can remove the variable to avoid confusion since it is unused.

Attempt to fix RVFI for instruction fetch exceptions. 

Generate correct cause for AMO faults