Rollup merge of #144354 - rafaeling:fix-142726-qnx8-link-fail, r=tgross35

fix(std): Fix undefined reference to __my_thread_exit on QNX 8.0

When cross-compiling for the x86_64/aarch64-unknown-nto-qnx800 target (QNX SDP 8.0), the build fails during the final link stage with the error:
```
error: linking with `qcc` failed: exit status: 1
  ...
  = note: undefined reference to `__my_thread_exit'
 ```

- **On QNX 7.1**: The __my_thread_exit symbol is defined and exported by the main C library (libc.a/libc.so). The std backtrace code can therefore successfully take its address at compile time.

- **On QNX 8.0**: As part of a toolchain modernization, this symbol has been refactored. It is no longer present in any of the standard system libraries (.a or .so).

This patch addresses the problem at its source by conditionally compiling the problematic code.

Fixes rust-lang/rust#142726

Author: Zalathar