Rollup merge of #140367 - folkertdev:asm-cfg, r=nnethercote
add `asm_cfg`: `#[cfg(...)]` within `asm!`
tracking issue: https://github.com/rust-lang/rust/issues/140364
blocked on: https://github.com/rust-lang/rust/pull/140490
This feature was discussed in https://github.com/rust-lang/rust/issues/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 https://github.com/rust-lang/rust/pull/140490, which should be merged first.
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