`asm!` lints based on `cfg` rather than target

[tgross35]

For example:

#[cfg(target_arch = "x86")]
fn foo() {
    unsafe { asm!("0: jmp 0b") };
}

This currently raises binary_asm_labels only if targeting x86. Ideally we would be able to raise the lint regardless of what the current target is.

Requested in #127821 and #127935

[tgross35]

@estebank I'm a bit light on the details here, and not sure I understand it fully. Do we do anything like this at the current time? It would be great if we could validate assembly for all targets (actual correctness, not just lints), but block-level cfg seems to make that difficult.

[tgross35]

Oh, did you mean if cfg!(...) rather than attribute config? That seems potentially easier, but I have no clue what level of support these things have.

[tgross35]

More info in #127935 (comment)