This fixes another bug like
#1197: `_` is no puctuation --
another case of dtolnay/proc-macro2#475,
fall-out of dtolnay/proc-macro2#470.

## Testing

This enables building RIOT sources again without `--locked`. I'm not
entirely sure, but I think the offending assembly was

```c
  __ASM ("rev %0, %1" : __CMSIS_GCC_OUT_REG (result) : __CMSIS_GCC_USE_REG (value) );
```

which gets turned into

```rust
    asm!(
        "rev {0}, {1}", lateout(reg) result, inlateout(reg) value => _,
        options(preserves_flags, pure, readonly)
    );
```

where the `_` is the identifier (formerly punctuation) which c2rust
trips over.