Only allow x86_64 for OS X builds
Only allow x86_64 for OS X builds

This prevents the OS X target from being picked up during iOS simulator
builds (as seen in
https://github.com/github/Archimedes/pull/34#issuecomment-57824037).

Quick/Nimble,alexbasson/Nimble,nikita-leonov/Nimble,jeffh/Nimble,joemasilotti/Nimble,octo-technology/Nimble,makezwl/zhao,jrmiddle/Nimble,alexbasson/Nimble,briancroom/Nimble,romankl/Nimble,romankl/Nimble,NatashaTheRobot/Nimble,octo-technology/Nimble,alvinvarghese/Nimble,feinstruktur/Nimble,Quick/Nimble,gfontenot/Nimble,dgdosen/Nimble,phatblat/Nimble,jeffh/Nimble,NatashaTheRobot/Nimble,DanielAsher/Nimble,nikita-leonov/Nimble,twobitlabs/Nimble,alexbasson/Nimble,alexbasson/Nimble,alvinvarghese/Nimble,twobitlabs/Nimble,mrackwitz/Nimble,NatashaTheRobot/Nimble,mishimay/Nimble,mishimay/Nimble,abbeycode/Nimble,rbeeger/Nimble,octo-technology/Nimble,briancroom/Nimble,joemasilotti/Nimble,mrackwitz/Nimble,jrmiddle/Nimble,abbeycode/Nimble,vtongcoder/Nimble,abbeycode/Nimble,mrackwitz/Nimble,rbeeger/Nimble,github/Nimble,mrackwitz/Nimble,vtongcoder/Nimble,NatashaTheRobot/Nimble,ashfurrow/Nimble,DanielAsher/Nimble,msdgwzhy6/Nimble,Quick/Nimble,mishimay/Nimble,jrmiddle/Nimble,jrmiddle/Nimble,joemasilotti/Nimble,DanielAsher/Nimble,dgdosen/Nimble,gfontenot/Nimble,msdgwzhy6/Nimble,gfontenot/Nimble,msdgwzhy6/Nimble,makezwl/zhao,abbeycode/Nimble,msdgwzhy6/Nimble,alvinvarghese/Nimble,romankl/Nimble,feinstruktur/Nimble,twobitlabs/Nimble,makezwl/zhao,briancroom/Nimble,ScottRobbins/Nimble,gfontenot/Nimble,dgdosen/Nimble,alvinvarghese/Nimble,phatblat/Nimble,dgdosen/Nimble,ScottRobbins/Nimble,briancroom/Nimble,AnthonyMDev/Nimble,octo-technology/Nimble,nikita-leonov/Nimble,romankl/Nimble,rbeeger/Nimble,rbeeger/Nimble,ashfurrow/Nimble,phatblat/Nimble,github/Nimble,AnthonyMDev/Nimble,AnthonyMDev/Nimble,jeffh/Nimble,jeffh/Nimble,ScottRobbins/Nimble,feinstruktur/Nimble,ashfurrow/Nimble,vtongcoder/Nimble