rustc-builtins: use the target *specification* (rather than its triple) in build.rs

[japaric]

follow up of #35021

rustc-builtins' build.rs uses $TARGET to decide which intrinsics to compile. This won't work well with custom targets because these targets can have arbitrary triples (e.g. cortex-m0) that don't reflect the actual target specification (ARM architecture, no OS, etc). To make rustc-builtins more robust we should use the target specification (target_arch, target_os, etc) instead of the triple. This will improve rustc-builtins's support of custom targets and should simplify its build.rs logic: if spec.target_arch == "arm" instead of if target.starts_with("arm") || target.starts_with("thumb").

There's no standard mechanism to access the target specification from a build script but we could use an external crate like @nagisa's target_build_utils.

cc @alexcrichton @brson

[nagisa]

I was planning to write an RFC on the topic of built-in and custom targets overall. I’ll try to get to it once I finish my other in-progress stuff.

[alexcrichton]

Would certainly be nice to have! This logic should probably all live in gcc-rs anyway, and I wouldn't mind piling on more parsing/configuration to that crate, but if @nagisa's got an RFC along these lines sounds good to me to wait for that as well

[japaric]

TIL: rustc --print cfg is a thing. We could use that.

[brson]

rustc --print cfg seems fine.

[Mark-Simulacrum]

From #36512 (comment), I believe this has landed "upstream" in the compiler-builtins repo, so I'm going to close this issue since it's not really tracking anything. For those interested, rust-lang/compiler-builtins#66 is probably the best place to watch.