[build] Verify a host is supported when setting build options
[build] Verify a host is supported when setting build options

By verifying that the host is supported before doing any operations, we can
assume that the host value is properly formed, meaning we can simplify the logic
to set values based on the host. Follow-up commits will do exactly that.

jmgc/swift,ahoppen/swift,allevato/swift,glessard/swift,tkremenek/swift,harlanhaskins/swift,CodaFi/swift,aschwaighofer/swift,CodaFi/swift,atrick/swift,atrick/swift,allevato/swift,parkera/swift,jckarter/swift,karwa/swift,jckarter/swift,lorentey/swift,apple/swift,ahoppen/swift,gregomni/swift,allevato/swift,hooman/swift,ahoppen/swift,jmgc/swift,gregomni/swift,stephentyrone/swift,atrick/swift,xwu/swift,jmgc/swift,harlanhaskins/swift,allevato/swift,nathawes/swift,apple/swift,hooman/swift,JGiola/swift,ahoppen/swift,rudkx/swift,airspeedswift/swift,parkera/swift,atrick/swift,lorentey/swift,glessard/swift,atrick/swift,aschwaighofer/swift,hooman/swift,aschwaighofer/swift,gribozavr/swift,airspeedswift/swift,xwu/swift,ahoppen/swift,benlangmuir/swift,airspeedswift/swift,nathawes/swift,CodaFi/swift,JGiola/swift,nathawes/swift,gribozavr/swift,gregomni/swift,lorentey/swift,harlanhaskins/swift,stephentyrone/swift,glessard/swift,apple/swift,benlangmuir/swift,tkremenek/swift,hooman/swift,aschwaighofer/swift,hooman/swift,lorentey/swift,xwu/swift,CodaFi/swift,airspeedswift/swift,karwa/swift,rudkx/swift,xwu/swift,JGiola/swift,aschwaighofer/swift,jckarter/swift,JGiola/swift,jckarter/swift,stephentyrone/swift,parkera/swift,karwa/swift,gregomni/swift,aschwaighofer/swift,karwa/swift,jckarter/swift,atrick/swift,lorentey/swift,rudkx/swift,lorentey/swift,roambotics/swift,nathawes/swift,nathawes/swift,benlangmuir/swift,hooman/swift,jmgc/swift,rudkx/swift,gribozavr/swift,tkremenek/swift,gribozavr/swift,tkremenek/swift,airspeedswift/swift,harlanhaskins/swift,allevato/swift,lorentey/swift,glessard/swift,benlangmuir/swift,karwa/swift,tkremenek/swift,roambotics/swift,harlanhaskins/swift,parkera/swift,rudkx/swift,stephentyrone/swift,roambotics/swift,airspeedswift/swift,stephentyrone/swift,xwu/swift,JGiola/swift,tkremenek/swift,karwa/swift,apple/swift,roambotics/swift,roambotics/swift,jckarter/swift,gribozavr/swift,karwa/swift,allevato/swift,gribozavr/swift,benlangmuir/swift,jmgc/swift,parkera/swift,apple/swift,nathawes/swift,rudkx/swift,tkremenek/swift,parkera/swift,ahoppen/swift,jmgc/swift,gregomni/swift,roambotics/swift,CodaFi/swift,harlanhaskins/swift,parkera/swift,gregomni/swift,hooman/swift,jmgc/swift,JGiola/swift,lorentey/swift,jckarter/swift,parkera/swift,glessard/swift,gribozavr/swift,xwu/swift,gribozavr/swift,aschwaighofer/swift,glessard/swift,allevato/swift,xwu/swift,apple/swift,CodaFi/swift,stephentyrone/swift,stephentyrone/swift,airspeedswift/swift,CodaFi/swift,benlangmuir/swift,karwa/swift,harlanhaskins/swift,nathawes/swift