Add MRET and WFI; fix bug in compressed branch/jump instruction emission

[Keybinder]

This PR adds MRET and WFI instructions, which I believe should be implemented for any base ISA, since it's expected that all M-mode instructions are implemented. objdump disassembles both correctly when assembling with the new code.

Additionally, it fixes a bug where instructions like c.jal fn, c.bnez a0, label emit incorrect offsets, since the PC-relative handling that's done for the base branch/jump instructions wasn't done for the compressed ones. This wasn't caught by the C extension tests since they only test on constant immediates rather than labels in the context of a program. Full integration tests (i.e. assemble a more complex program that uses multiple extensions and check it matches a binary exactly) might be good.

P.S. It's a good assembler and I'm having fun using it.

[theandrew168]

Thanks for giving Bronzebeard a try and for opening this PR! I originally only implemented the unprivileged instructions but see value in also adding the privileged ones. I'm going to add a few more tests and then I'll get this merged.

EDIT: Actually, I'd like to add the rest of the privileged instructions so I'm going to merge this and add the rest (plus more tests) as a follow-up.