Derive xlen values from a logarithmic constant

[Timmmm]

The motivation for this is that it makes deriving CHERI constants (field widths etc.) which vary between RV32 and RV64 a lot easier.

[github-actions[bot]]

Test Results

712 tests  ±0   712 :white_check_mark: ±0   0s :stopwatch: ±0s   6 suites ±0     0 :zzz: ±0    1 files   ±0     0 :x: ±0 

Results for commit d1c34783. ± Comparison against base commit a58c58c6.

:recycle: This comment has been updated with latest results.

[arichardson]

Sounds like this needs a new sail version? Would be good if there was a new sail release soon (@Alasdair is this planned soon?) and we can add a version check (#370).

[Timmmm]

Yeah I think it's because CI doesn't pin the version of the Sail compiler we use, so when the newer version was released which supports these types it magically fixed this PR even though I didn't change anything.

#532 will pin the Sail compiler version.

[jordancarlin]

Probably worth merging an updated version of #370 before we merge this to prevent issues for anyone still running Sail 0.17. That way they will at least get a descriptive error message. Might also be worth getting #532 finished so that when people update, they can use the new binary installation.