build: allow the user to specify the llvm tools
build: allow the user to specify the llvm tools

This is needed for cross-compiling the toolchain to Windows ARM64 on
Windows x64 using Visual Studio which does not permit the NATIVE
sub-build to work properly.

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