... stacktrace into a file on non-Win32 systems.
This has a few advantages:
- First, we don't need to duplicate stacktrace code inside the
  independent gimp-debug-tool (I even noticed that the version in the
  tool was gdb-only and not updated for lldb fallback; proof that code
  duplication is evil!). Instead, even on a crash, we can create the
  stacktrace from the main binary and simply pass it as a file.
- Secondly, that allows to fallback to the backtrace() API even for
  crashes (this was not possible if the backtrace was done from a
  completely different process). That's nice because this makes that we
  will always get backtraces in Linux (even though backtrace() API is
  not as nice as gdb/lldb, it's better than nothing).
- Finally this makes the code smaller (i.e. easier to maintain), more
  consistent and similar on all platforms.