system-id.mk: fixed 64bit detection

64-bit systems were not detected correctly, causing the download of a wrong Qt binary which didn't install with the following error:

Error during installation process (com.nokia.ndk.tools.harmattan.sysroot):
Execution failed(Unexpected exit code: 1): "/home/trooper/Desktop/devel/TauLabs/tools/qtsdk-v1.2.1/Madde/postinstall/harmattan-postinstall.sh"