make abort intrinsic safe, and correct its documentation
Turns out `std::process::abort` is not the same as the intrinsic, the comment was just wrong. Quoting from the unix implementation:
```
// On Unix-like platforms, libc::abort will unregister signal handlers
// including the SIGABRT handler, preventing the abort from being blocked, and
// fclose streams, with the side effect of flushing them so libc buffered
// output will be printed. Additionally the shell will generally print a more
// understandable error message like "Abort trap" rather than "Illegal
// instruction" that intrinsics::abort would cause, as intrinsics::abort is
// implemented as an illegal instruction.
```
34cce58d81