Fix #2515: use better way to detect 64-bit OS.
Fix #2515: use better way to detect 64-bit OS.

andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons