Rollup merge of #147736 - folkertdev:stabilize-asm-cfg, r=jdonszelmann

Stabilize `asm_cfg`

tracking issue: https://github.com/rust-lang/rust/issues/140364
closes https://github.com/rust-lang/rust/issues/140364

Reference PR:

- https://github.com/rust-lang/reference/pull/2063

# Request for Stabilization

## Summary

The `cfg_asm` feature allows `#[cfg(...)]` and `#[cfg_attr(...)]` on  the arguments of the assembly macros, for instance:

```rust
asm!( // or global_asm! or naked_asm!
    "nop",
    #[cfg(target_feature = "sse2")]
    "nop",
    // ...
    #[cfg(target_feature = "sse2")]
    a = const 123, // only used on sse2
);
```

## Semantics

Templates, operands, `options` and `clobber_abi` in the assembly macros (`asm!`, `naked_asm!` and `global_asm!`) can be annotated with `#[cfg(...)]` and `#[cfg_attr(...)]`. When the condition evaluates to true, the annotated argument has no effect, and is completely ignored when expanding the assembly macro.
## Documentation

reference PR: https://github.com/rust-lang/reference/pull/2063

## Tests

- [tests/ui/asm/cfg.rs](https://github.com/rust-lang/rust/blob/master/tests/ui/asm/cfg.rs) checks that `cfg`'d arguments where the condition evaluates to false have no effect
- [tests/ui/asm/cfg-parse-error.rs](https://github.com/rust-lang/rust/blob/master/tests/ui/asm/cfg.rs) checks the parsing rules (parsing effectively assumes that the cfg conditions are all true)

## History

- https://github.com/rust-lang/rust/issues/140279
- https://github.com/rust-lang/rust/pull/140367

# Resolved questions

**how are other attributes handled**

Other attributes are parsed,  but explicitly rejected.

# unresolved questions

**operand before template**

The current implementation expects at least one template string before any operands. In the example below, if the `cfg` condition evaluates to true, the assembly block is ill-formed. But even when it evaluates to `false` this block is rejected, because the parser still expects just a template (a template is parsed as an expression and then validated to ensure that it is or expands to a string literal).

Changing how this works is difficult.
```rust
// This is rejected because `a = out(reg) x` does not parse as an expresion.
asm!(
	#[cfg(false)]
	a = out(reg) x, //~ ERROR expected token: `,`
	"",
);
```

**lint on positional arguments?**

Adding a lint to warn on the definition or use of positional arguments being `cfg`'d out was discussed in https://github.com/rust-lang/rust/issues/140279#issuecomment-2832237372 and subsequent comments. Such a lint is not currently implemented, but that may not be a blocker based on the comments there.

r? `@traviscross` (I'm assuming you'll reassign as needed)