Rollup merge of #140367 - folkertdev:asm-cfg, r=nnethercote

add `asm_cfg`: `#[cfg(...)]` within `asm!`

tracking issue: rust-lang/rust#140364
blocked on: rust-lang/rust#140490

This feature was discussed in rust-lang/rust#140279. It allows configuring templates and operands in the assembly macros, for example:

```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
);
```

r? `@tgross35`

cc `@traviscross` `@Amanieu`

Now builds on rust-lang/rust#140490, which should be merged first.

Author: tgross35