Rollup merge of #85967 - atopia:update-l4re-target, r=petrochenkov

add support for the l4-bender linker on the x86_64-unknown-l4re-uclibc tier 3 target

This PR contains the work by ```@humenda``` to update support for the `x86_64-unknown-l4re-uclibc` tier 3 target (published at [humenda/rust](https://github.com/humenda/rust)), rebased and adapted to current rust in follow up commits by myself. The publishing of the rebased changes is authorized and preferred by the original author. As the goal was to distort the original work as little as possible, individual commits introduce changes that are incompatible to the newer code base that the changes were rebased on. These incompatibilities have been remedied in follow up commits, so that the PR as a whole should result in a clean update of the target.
If you prefer another strategy to mainline these changes while preserving attribution, please let me know.

Author: matthiaskrgr

panic_unwind/src/lib.rs

@@ -39,6 +39,10 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_os = "hermit")] {
         #[path = "hermit.rs"]
         mod real_imp;
+    } else if #[cfg(target_os = "l4re")] {
+        // L4Re is unix family but does not yet support unwinding.
+        #[path = "dummy.rs"]
+        mod real_imp;
     } else if #[cfg(target_env = "msvc")] {
         #[path = "seh.rs"]
         mod real_imp;