Auto merge of #44251 - kennytm:osx-backtrace, r=alexcrichton

Add libbacktrace support for Apple platforms (resubmitted)

Resubmitting #43422 rebased on the current master (cc @JohnColanduoni).

I have added an additional commit to fallback to `dladdr`-based `resolve_symbol` if `libbacktrace` returns `None`, otherwise the stack trace will be full of `<unknown>` when you forget to pass the `-g` flag (actually it seems — at least on macOS — the `dladdr` symbol is more accurate than the `libbacktrace` one).

src/test/run-pass/backtrace-debuginfo.rs

@@ -33,9 +33,7 @@ macro_rules! dump_and_die {
     ($($pos:expr),*) => ({
         // FIXME(#18285): we cannot include the current position because
         // the macro span takes over the last frame's file/line.
-        if cfg!(any(target_os = "macos",
-                    target_os = "ios",
-                    target_os = "android",
+        if cfg!(any(target_os = "android",
                     all(target_os = "linux", target_arch = "arm"),
                     target_os = "freebsd",
                     target_os = "dragonfly",