Empty stub of the probe ABI detector.

Apart from the dummy implementation (which is functionally equivalent to
the current behavior), we can't actually detect anything yet though, on
any platform.
Add convenience API to check if the Qt version is set.
Add caching of probe ABIs for QtCore DLLs.
Implement QtCore version detection for ELF, and detecting QtCore for a PID.

We currently have two ways of detecting the version:
- the file name
- the version printed when executing QtCore.so

For detecting QtCore for a given PID we only support /proc lookup at the
moment, a non-/proc fallback as we have it in the process list still needs
to be added.
Add architecture detection for ELF.
Add ldd-based detection of QtCore DLL used by a executable.