Debugger: Rework dumper detection

Recent versions of GDB seem to require 'make install' to find their
Python bits. Given that this is not really an option, check the
usual suspects, too.

Change-Id: I5217c0184681d4a86992fe0b8989498843b26cea
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>