64-bit MinGW recognized as 32-bit

I'm using a 64-bit gcc 4.8.1 on windows (w64, seh, 4.8.1, rev. 5, posix from MinGW-builds: http://sourceforge.net/projects/mingwbuilds/).

This compiler returns the ABI-string
x86_64-w64-mingw32
when calling
g++ -dumpmachine

Unfortunately this is recognized as a 32-bit compiler by QtCreator, as shown in the attatched image.

I checked now the toolchains.xml file ... and it actually lists both ABIs (32&64 bit, I'm attatching the file for full info):

<data>
<variable>ToolChain.3</variable>
<valuemap type="QVariantMap">
<value type="QString" key="ProjectExplorer.GccToolChain.Path">C:/development/mingw-builds/x64-4.8.1-posix-seh-rev5/mingw64/bin/x86_64-w64-mingw32-g++.exe</value>

...

<valuelist type="QVariantList" key="ProjectExplorer.GccToolChain.SupportedAbis">
<value type="QString">x86-windows-msys-pe-64bit</value>
<value type="QString">x86-windows-msys-pe-32bit</value>
</valuelist>
<value type="QString" key="ProjectExplorer.GccToolChain.TargetAbi">x86-windows-msys-pe-64bit</value>

...

</valuemap>
</data>

But these versions are not displayed in the according combobox in the creator. and I can't select the 64-bit ABI, which seems to even be the default setting (that's how I would interpret ProjectExplorer.GccToolChain.TargetAbi above).