cranelift-assembler-x64: no_std support

[the-ssd]

Part of #12222, slightly refactored. Related to #1158.

Adds support for no_std compilation of cranelift-assmebler-x64. Required for cranelift-codegen to support no_std compilation.

[the-ssd]

I guess the CI is running cargo test with all features, should I change core feature to std, or can you help me fix the CI?

[alexcrichton]

Thanks for this!

[the-ssd]

@alexcrichton Thank you too! This is almost everything needed for #12222. I still am not sure if the way I deal with OnceLock is good enough to be merged (because it just recomputes MachineEnv each time). And some architectures use powi which libm doesn't have (only x86 and arm compile with no_std).

[alexcrichton]

Want to rebase that PR after the recent merges and we can brainstorm over there? Should in theory be a bit easier to think about with a smaller diff

[the-ssd]

@alexcrichton I have already rebased it, a lot of it is are one line changes like adding prelude, fixing hashmap import (also FxHashMap on no_std now works). Really the only thing is OnceLock. I removed it, but now it's recomputed each time. I can try to find a no_std replacement or move some stuff around in order to use sync_nostd.rs like suggested in the old PR.

Also I accidentally formated Cargo.toml, so maybe I should revert it