Fix passing target-cpu and codegen-units to dependencies

[zheland]

Fixes #355

Changes

This PR fixes passing target-cpu to the dependencies using RUSTFLAGS instead of cargo rustc args.

Other arguments

• user-supplied codegen flags - okay with cargo rustc args.
• asm syntax - okay with cargo rustc args.
• target-cpu - should be passed as RUSTFLAGS to compile deps with target-cpu.
• debuginfo - probably don't affect deps, so okay with cargo rustc args.
• codegen-units - probably should be passed as RUSTFLAGS to be applicable for deps, but I am not sure. Please check this. If passed with RUSTFLAGS may overwrite user-specified codegen-units in RUSTFLAGS.

Notes

• RUSTFLAGS are prioritized over cargo rustc args, so cargo asm --native or cargo asm --target-cpu=CPU will now overwrite user-specified target-cpu in RUSTFLAGS.
• Using write!(...).unwrap(); to &mut String seems to be fine and already used in code.

[pacak]

Makes sense. Initially I thought that it doesn't really matter how dependencies are compiled, but your example with benchmarks is pretty convincing.

As far as codegen units are concerned - I'm setting them to 1 just to make life easier figuring out what .s file we should be using. I submitted changes to rustc (rust-lang/rust#122597), but things are stuck behind cargo giving access to this info (rust-lang/cargo#13672). There are some flags to cargo that would make it print the right invocation of rustc so we can run it directly and access the output.

debuginfo - used to attribute asm lines to rust lines, won't change the compilation result...

Anyway, I think your change is good to be merged as is. Thanks again for your contribution ❤️