LLVM lto removes `__INTERRUPTS` due to missing `#[used]` attribute

[Ddystopia]

Hello, I compiled my code base with fat lto and __INTERRUPTS went missing - event KEEP didn't help as the symbol got eliminated before, only "link-arg=-Wl,--undefined=__INTERRUPTS" helped.

IIUC adding #[used] should never be harmful, so users without LTO will not see the difference.

[burrbull]

Feel free to open PR.