Building Rustc on m68k

I am currently working on building rustc for m68k(since that seems like a more achievable short-term goal). Many of the roadblocks I'll hit on m68k will also likely be shared with other targets - this issue could server as a reference for that.

Here is a list of currently-known issues:

• - linux-raw-sys does not support m68k. This crate is needed by rustc, and a lot of other crates. Proper support will require a moderate amount of work, but it something that can be done now.

• - The rustix crate does not support m68k. (more specifically, the iocl::const module needs the right values for syscall arguments).

• - The const assert in https://github.com/rust-lang/rust/blob/ce5fdd7d42aba9a2925692e11af2bd39cf37798a/compiler/rustc_data_structures/src/tagged_ptr.rs#L163 fails. EDIT: the problem is the alignment of usize. Rust uses those bits to store type tags. If the aligement is less than 4, this fails. I have a fix ready to be sent & merged.

• - The bootstrap script fails to compile native GCC for m68k. This prevents it from building any codegen backends or tools later down the line. This is skipable with some fiddling.

• - When building other tools(eg. cargo) the bootstrap script "forgets" where the std for m68k is.

So, as of toady, with some workarounds, I can get pretty far into the cross-compilation process - a bit more work, and we may be able to get rustc running on m68k.

@glaubitz I think this issue might be of interest to you.

Start by reviewing the listed m68k blockers: linux-raw-sys, rustix, compiler/rustc_data_structures/src/tagged_ptr.rs, and the bootstrap script. Check the current status of the tagged_ptr alignment fix and the m68k support gaps, then reproduce the bootstrap failures. Done means rustc and its tools can be built for m68k without the listed workarounds.