fix(std): Add __my_thread_exit stub for QNX 8

This commit adds an empty stub for the  function
for QNX 8 targets. This symbol is required by the unwinder but is
not present, causing a linking failure when building with the
standard library.

Author: rafaeling

library/std/src/sys/backtrace.rs

@@ -113,7 +113,7 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
                         res = bt_fmt.frame().symbol(frame, symbol);
                     }
                 });
-                #[cfg(target_os = "nto")]
+                #[cfg(all(target_os = "nto", not(target_env = "nto80")))]
                 if libc::__my_thread_exit as *mut libc::c_void == frame.ip() {
                     if !hit && print {
                         use crate::backtrace_rs::SymbolName;