Mstatus SXL/UXL issues

[Timmmm]

Just so we don't forget, #652 accidentally made mstatus.SXL/UXL non-zero on RV32 which is incorrect. Fix in #683

Also an existing issue is that they should be read-only zero if the corresponding mode is not implemented. That will be fixed in a future PR.

[radimkrcmar]

I already have the patches for both issues. No idea how to write tests for them, though.

[Timmmm]

Tests, shmests... :-D

Once the CMake PR is merged I'm going to add support for Sail unit tests so we can at least do some testing of things.

Both of those things should probably actually be ELF tests rather than unit tests but getting that up and running is way more work, and adds a RISC-V compiler as a dependency which would be nice to avoid if possible.

I almost think this domain needs a new programming language that is like "high level assembly" with a compiler & linker that are very easy to distribute (single statically linked executable). C and assembly are both rubbish for low level tests and benchmarks.

[Timmmm]

Both issues should be fixed now via #597.

I'm working on adding nice unit tests support to Sail to test this sort of thing: rems-project/sail#928