Audit uses of static mut

[ChrisDenton]

The new warning is causing CI to fail. E.g:

error: creating a mutable reference to mutable static is discouraged
   --> src/symbolize/gimli.rs:368:11
    |
368 |         f(MAPPINGS_CACHE.get_or_insert_with(Cache::new))
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable reference to mutable static
    |
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html>
    = note: mutable references to mutable statics are dangerous; it's undefined behavior if any other pointer to the static is used or if any other reference is created for the static while the mutable reference lives
    = note: `-D static-mut-refs` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(static_mut_refs)]`

error: could not compile `backtrace` (lib) due to 1 previous error

Either we should allow the lint and add some comments or else actually fix the issue.