qtcreator_process_stub is no longer found: Debugging in terminal fails

Qt Creator tries to start "qtcreator_process_stub" next to the binary of the running application in Utils::consoleprocess_*.cpp. This fails since the binary is now in the libexec directory.

The libexec path should also be configurable. E.g. archlinux has no libexec directory and uses subdirectories in lib instead.