Here's the log file from the Build & Run tab after building debug helpers in 32bit mode:

Building helper(s) with toolchain 'GCC (x86 32bit)' ...
Building helper 'GDB helper' in /Applications/Qt Creator v2.4.app/Contents/qtc-debugging-helper/46196080/

Running /Volumes/Data/QtCommercial/builds/qt-480/bin/qmake -spec macx-g++ dumper.pro -nocache ...

Running /usr/bin/make all -k ...
g++ -c -pipe -g -gdwarf-2 -Wall -W -fPIC -DUSE_QT_GUI=1 -DHAS_QOBJECT_P_H -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/Volumes/Data/QtCommercial/sources/qt-480-src/mkspecs/macx-g++ -I. -I/Volumes/Data/QtCommercial/builds/qt-480/lib/QtCore.framework/Versions/4/Headers -I/Volumes/Data/QtCommercial/builds/qt-480/include/QtCore -I/Volumes/Data/QtCommercial/builds/qt-480/lib/QtGui.framework/Versions/4/Headers -I/Volumes/Data/QtCommercial/builds/qt-480/include/QtGui -I/Volumes/Data/QtCommercial/builds/qt-480/include -I. -F/Volumes/Data/QtCommercial/builds/qt-480/lib -o dumper.o dumper.cpp
rm -f libdumper.1.0.0.dylib libdumper.dylib libdumper.1.dylib libdumper.1.0.dylib
g++ -headerpad_max_install_names -single_module -dynamiclib -compatibility_version 1.0 -current_version 1.0.0 -install_name libdumper.1.dylib -o libdumper.1.0.0.dylib dumper.o -F/Volumes/Data/QtCommercial/builds/qt-480/lib -L/Volumes/Data/QtCommercial/builds/qt-480/lib -framework QtGui -L/Volumes/Data/QtCommercial/builds/qt-480/lib -F/Volumes/Data/QtCommercial/builds/qt-480/lib -framework QtCore
ln -s libdumper.1.0.0.dylib libdumper.dylib
ln -s libdumper.1.0.0.dylib libdumper.1.dylib
ln -s libdumper.1.0.0.dylib libdumper.1.0.dylib
Build succeeded.

Build was succesfull but if I check the architecture from libdumper.1.0.0.dylib
with command: file libdumper.1.0.0.dylib

I'll get

libdumper.1.0.0.dylib: Mach-O 64-bit dynamically linked shared library x86_64