Rollup merge of #125920 - bjorn3:allow_static_mut_linkage_def, r=Urgau

Allow static mut definitions with #[linkage]

Unlike static declarations with #[linkage], for definitions rustc doesn't rewrite it to add an extra indirection.

This was accidentally disallowed in rust-lang/rust#125046.

cc rust-lang/rust#125800 (comment)

Author: workingjubilee