Fix issue13199 i386/iphone inversion of arguments in atomic swap MACRO

I don't know in which case mixing i386 & TARGET_OS_IS_IPHONE applies, but we cannot let this stinky inversion in place IMO...
See https://pharo.fogbugz.com/f/cases/13199/Stinky-argument-inversion-of-atomic-swap-MACRO-for-iphone-gcc-branch