adapt L4Bender implementation

- Fix style errors.

- L4-bender does not yet support dynamic linking.

- Stack unwinding is not yet supported for x86_64-unknown-l4re-uclibc.
  For now, just abort on panics.

- Use GNU-style linker options where possible. As suggested by review:
    - Use standard GNU-style ld syntax for relro flags.
    - Use standard GNU-style optimization flags and logic.
    - Use standard GNU-style ld syntax for --subsystem.

- Don't read environment variables in L4Bender linker. Thanks to
  CARGO_ENCODED_RUSTFLAGS introduced in rust-lang#9601, l4-bender's arguments can
  now be passed from the L4Re build system without resorting to custom
  parsing of environment variables.

Author: atopia

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;