Use g++.exe as toolchain path

.. for mingw, <blah>gcc.exe was emitted as
ProjectExplorer.GccToolChain.Path, but this
causes qbs plugin projects to fail to link
C++ applications correctly.

Change-Id: Ie7b772db33a500ce564adbc6c4561162afcdecd9
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>