cmake: Don't generate targets for empty sdk/arch combinations.
cmake: Don't generate targets for empty sdk/arch combinations.

See rdar://31642132.

sschiau/swift,gregomni/swift,danielmartin/swift,felix91gr/swift,frootloops/swift,gribozavr/swift,atrick/swift,uasys/swift,shahmishal/swift,alblue/swift,zisko/swift,deyton/swift,stephentyrone/swift,frootloops/swift,hughbe/swift,arvedviehweger/swift,brentdax/swift,tkremenek/swift,sschiau/swift,swiftix/swift,huonw/swift,xedin/swift,tkremenek/swift,gribozavr/swift,hooman/swift,airspeedswift/swift,calebd/swift,stephentyrone/swift,aschwaighofer/swift,austinzheng/swift,huonw/swift,lorentey/swift,stephentyrone/swift,xwu/swift,benlangmuir/swift,huonw/swift,ahoppen/swift,shajrawi/swift,OscarSwanros/swift,alblue/swift,Jnosh/swift,zisko/swift,benlangmuir/swift,jopamer/swift,nathawes/swift,parkera/swift,milseman/swift,glessard/swift,milseman/swift,aschwaighofer/swift,huonw/swift,allevato/swift,Jnosh/swift,frootloops/swift,harlanhaskins/swift,arvedviehweger/swift,swiftix/swift,practicalswift/swift,jckarter/swift,atrick/swift,harlanhaskins/swift,tjw/swift,milseman/swift,jmgc/swift,nathawes/swift,JGiola/swift,OscarSwanros/swift,austinzheng/swift,shajrawi/swift,atrick/swift,shajrawi/swift,rudkx/swift,gribozavr/swift,brentdax/swift,return/swift,CodaFi/swift,ahoppen/swift,swiftix/swift,deyton/swift,djwbrown/swift,jopamer/swift,tkremenek/swift,lorentey/swift,xwu/swift,gregomni/swift,natecook1000/swift,hooman/swift,harlanhaskins/swift,frootloops/swift,danielmartin/swift,natecook1000/swift,alblue/swift,rudkx/swift,felix91gr/swift,parkera/swift,devincoughlin/swift,shahmishal/swift,brentdax/swift,milseman/swift,apple/swift,sschiau/swift,sschiau/swift,arvedviehweger/swift,shahmishal/swift,swiftix/swift,JGiola/swift,parkera/swift,apple/swift,nathawes/swift,alblue/swift,nathawes/swift,amraboelela/swift,practicalswift/swift,xedin/swift,djwbrown/swift,jmgc/swift,harlanhaskins/swift,gregomni/swift,allevato/swift,JGiola/swift,Jnosh/swift,tkremenek/swift,Jnosh/swift,frootloops/swift,xwu/swift,airspeedswift/swift,calebd/swift,karwa/swift,return/swift,aschwaighofer/swift,hooman/swift,jmgc/swift,gribozavr/swift,danielmartin/swift,glessard/swift,calebd/swift,roambotics/swift,natecook1000/swift,rudkx/swift,airspeedswift/swift,stephentyrone/swift,shahmishal/swift,xedin/swift,milseman/swift,return/swift,hooman/swift,arvedviehweger/swift,lorentey/swift,nathawes/swift,Jnosh/swift,jtbandes/swift,OscarSwanros/swift,jmgc/swift,danielmartin/swift,jopamer/swift,jckarter/swift,lorentey/swift,OscarSwanros/swift,zisko/swift,karwa/swift,roambotics/swift,zisko/swift,benlangmuir/swift,hooman/swift,djwbrown/swift,aschwaighofer/swift,amraboelela/swift,apple/swift,natecook1000/swift,CodaFi/swift,alblue/swift,OscarSwanros/swift,xedin/swift,airspeedswift/swift,stephentyrone/swift,tjw/swift,jckarter/swift,xwu/swift,xedin/swift,hughbe/swift,gregomni/swift,arvedviehweger/swift,shajrawi/swift,benlangmuir/swift,arvedviehweger/swift,return/swift,calebd/swift,stephentyrone/swift,karwa/swift,ahoppen/swift,austinzheng/swift,alblue/swift,OscarSwanros/swift,parkera/swift,parkera/swift,huonw/swift,harlanhaskins/swift,jtbandes/swift,sschiau/swift,milseman/swift,allevato/swift,devincoughlin/swift,tjw/swift,djwbrown/swift,sschiau/swift,tjw/swift,karwa/swift,devincoughlin/swift,jopamer/swift,tkremenek/swift,jtbandes/swift,sschiau/swift,calebd/swift,natecook1000/swift,danielmartin/swift,parkera/swift,hughbe/swift,amraboelela/swift,swiftix/swift,amraboelela/swift,alblue/swift,deyton/swift,djwbrown/swift,brentdax/swift,swiftix/swift,harlanhaskins/swift,glessard/swift,jtbandes/swift,jckarter/swift,return/swift,lorentey/swift,shahmishal/swift,benlangmuir/swift,jckarter/swift,stephentyrone/swift,danielmartin/swift,devincoughlin/swift,tjw/swift,danielmartin/swift,ahoppen/swift,hughbe/swift,hughbe/swift,practicalswift/swift,austinzheng/swift,rudkx/swift,xwu/swift,uasys/swift,calebd/swift,deyton/swift,lorentey/swift,uasys/swift,benlangmuir/swift,austinzheng/swift,practicalswift/swift,parkera/swift,shajrawi/swift,jmgc/swift,arvedviehweger/swift,aschwaighofer/swift,frootloops/swift,hughbe/swift,glessard/swift,milseman/swift,devincoughlin/swift,CodaFi/swift,natecook1000/swift,karwa/swift,deyton/swift,tjw/swift,allevato/swift,zisko/swift,huonw/swift,zisko/swift,jtbandes/swift,airspeedswift/swift,rudkx/swift,felix91gr/swift,allevato/swift,gribozavr/swift,jopamer/swift,swiftix/swift,xedin/swift,practicalswift/swift,shahmishal/swift,uasys/swift,CodaFi/swift,zisko/swift,aschwaighofer/swift,roambotics/swift,glessard/swift,shajrawi/swift,uasys/swift,atrick/swift,atrick/swift,jckarter/swift,airspeedswift/swift,natecook1000/swift,tkremenek/swift,apple/swift,roambotics/swift,Jnosh/swift,huonw/swift,uasys/swift,djwbrown/swift,xwu/swift,amraboelela/swift,amraboelela/swift,parkera/swift,gregomni/swift,amraboelela/swift,rudkx/swift,gribozavr/swift,shahmishal/swift,austinzheng/swift,roambotics/swift,shajrawi/swift,CodaFi/swift,devincoughlin/swift,apple/swift,practicalswift/swift,CodaFi/swift,hooman/swift,xedin/swift,glessard/swift,karwa/swift,brentdax/swift,JGiola/swift,deyton/swift,sschiau/swift,return/swift,gregomni/swift,hooman/swift,deyton/swift,roambotics/swift,return/swift,nathawes/swift,practicalswift/swift,ahoppen/swift,nathawes/swift,JGiola/swift,practicalswift/swift,jckarter/swift,jtbandes/swift,JGiola/swift,harlanhaskins/swift,Jnosh/swift,jopamer/swift,devincoughlin/swift,OscarSwanros/swift,jmgc/swift,tkremenek/swift,austinzheng/swift,allevato/swift,gribozavr/swift,airspeedswift/swift,ahoppen/swift,devincoughlin/swift,felix91gr/swift,apple/swift,lorentey/swift,brentdax/swift,hughbe/swift,lorentey/swift,brentdax/swift,CodaFi/swift,karwa/swift,uasys/swift,aschwaighofer/swift,djwbrown/swift,gribozavr/swift,jmgc/swift,tjw/swift,shahmishal/swift,allevato/swift,atrick/swift,felix91gr/swift,shajrawi/swift,felix91gr/swift,frootloops/swift,karwa/swift,xedin/swift,jtbandes/swift,felix91gr/swift,jopamer/swift,xwu/swift,calebd/swift