Correctly set toolchain prefix for GCC cross compilers.
Correctly set toolchain prefix for GCC cross compilers.

Change-Id: I3e1c73a22ff2abd9ce7d64515a2830aaf076095c
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

qtproject/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,villytiger/qbs,villytiger/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,Distrotech/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,villytiger/qbs