Partial support for no_std in cranelift_codegen

[the-ssd]

Used to close #1158, allows for cranelift to be used in a no_std environment, like a kernel.

There is already #9007, which stalled.

And I am at a point I think it makes sense to ask someone what do you want to be done about FxHashMap and OnceLock. There are alternative crates that implement it, like this one (also has a comparison chart).

What is left to do:

• isle codegen generates code that depends on no_std, solved in the previous PR (https://github.com/bytecodealliance/wasmtime/pull/9007/files#diff-0496ab91e0634653bd3d4295c7aa90e3f06b0c5f462332db0997a22f294cbf94)
• separate PRs for: cranelift_assembler_x86 (cranelift-assembler-x64: no_std support #12235), moving large parts to core and alloc (cranelift-codegen: rename most uses of std to core and alloc #12237), and isle (cranelift-isle: no_std support #12236)
• FxHashMap
• floating point issues
• OnceLock (sort of)
• CI
• libm: roundeven has a typo in it's implementation Fix a typo in libm::Libm::roundeven rust-lang/compiler-builtins#1053, so a hack is used, waiting for libm to be published to crates.io

[the-ssd]

I found something that might be useful.

https://rust-lang.github.io/rust-clippy/master/#std_instead_of_core
https://rust-lang.github.io/rust-clippy/master/#alloc_instead_of_core
https://rust-lang.github.io/rust-clippy/master/#std_instead_of_alloc

source

[the-ssd]

I've removed OnceLock, but MachineEnv is now recomputed each time.

[the-ssd]

core feature can be removed

[the-ssd]

Unsure if this is correct, function always returns 0

[alexcrichton]

cc @cfallin should this perhaps continue to skip this value? Or panic! since this should be unreachable? Or is 0 ok to leave in?

[cfallin]

Yes, if we don't have unwind then we don't have the appropriate metadata to allow a native debugger to interpret frames anyway, so we should continue. I'm a bit curious how this built in a non-unwind build before -- or was that broken because std always enabled unwind?

[the-ssd]

It was broken

[cfallin]

@the-ssd OK, yep, let's do a continue in this case.

[the-ssd]

Can you check if I did it correctly?
It just looks a bit wrong, like there has to be a better solution

[alexcrichton]

Looks good to me!

[tgross35]

• libm: roundeven has a typo in it's implementation Fix a typo in libm::Libm::roundeven rust-lang/compiler-builtins#1053, so a hack is used, waiting for libm to be published to crates.io

FYI since I noticed the crosslink, this was released in https://github.com/rust-lang/compiler-builtins/releases/tag/libm-v0.2.16