Fix detection of cdb path in qbs project file.

Change-Id: Ibd54982b371f320625ec4959367febc54dd6e79e
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Compile fix for the cdb extension.

The naming of the architecture subfolders have changed between the
"Debugging Tools for Windows" versions and the tools shipped with the
Windows 8 SDK.
Old names: amd64 and i386.
New names: x64 and x86.

Change-Id: Id828ba9fbb0c5074d1046ac7a710fedf07c6c984
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>