science/qt6-quick3dphysics: fix build on powerpc64*

Correctly detect 64-bits:
/wrkdirs/usr/ports/science/qt6-quick3dphysics/work/qtquick3dphysics-everywhere-src-6.5.3/src/3rdparty/PhysX/source/common/src/CmPtrTable.h:137:1: error: 'PxCompileTimeAssert_Dummy' declared as an array with a negative size
PX_COMPILE_TIME_ASSERT(sizeof(Cm::PtrTable)==8);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/wrkdirs/usr/ports/science/qt6-quick3dphysics/work/qtquick3dphysics-everywhere-src-6.5.3/src/3rdparty/PhysX/pxshared/include/foundation/PxPreprocessor.h:463:76: note: expanded from macro 'PX_COMPILE_TIME_ASSERT'

^~~~~~~~~~~~~~

1 error generated.