Support -C no-split-stack regardless of target triple

[kmcallister]

See #14394, which was closed on the grounds that this should be determined solely by the target triple.

I want -C no-split-stack for my experiments making really tiny Linux binaries in Rust. In this case the target is the usual x86_64-unknown-linux-gnu, and rustc is built in the usual way for the platform.

It also seems useful for writing C libraries in Rust. We won't necessarily have __morestack available, and people who are already using C are more likely to want to compromise on this particular facet of memory safety.

This is a pretty big pain point — I'm currently running sed on the LLVM IR to fix it! There may be a more involved solution but the original PR was closed months ago and I'd like to have the simple fix ASAP. If people are okay with -C no-split-stack, I could revive the code from the PR. Or we could make it something like --unsafe-codegen-options no-split-stack.

[mahkoh]

I'd also like this. The flexible target specification can theoretically solve this but afaik you have to manually copy and modify one file for every platform for which you want to compile. This switch would make this much easier.

[brson]

I'm ok with -C no-split-stack.