[build-script] Make sure that we build using an external compiler-rt on /all/ platforms, not just OS X.
[build-script] Make sure that we build using an external compiler-rt on /all/ platforms, not just OS X.

This is important for a few reasons:

1. It ensures that on all platforms we use the just built clang to compile
compiler-rt. This ensures that the clang used to compiler-rt has all of the
features necessary to compile compiler-rt in the face of changes in the
underlying implementation.

2. It ensures that all platforms build compiler-rt in the same manner.

ben-ng/swift,djwbrown/swift,karwa/swift,hughbe/swift,jopamer/swift,brentdax/swift,allevato/swift,kstaring/swift,JaSpa/swift,xedin/swift,milseman/swift,jopamer/swift,therealbnut/swift,gregomni/swift,tkremenek/swift,felix91gr/swift,lorentey/swift,karwa/swift,bitjammer/swift,uasys/swift,kperryua/swift,shahmishal/swift,gribozavr/swift,JaSpa/swift,zisko/swift,djwbrown/swift,uasys/swift,stephentyrone/swift,rudkx/swift,shajrawi/swift,zisko/swift,nathawes/swift,kstaring/swift,JGiola/swift,nathawes/swift,airspeedswift/swift,OscarSwanros/swift,hughbe/swift,bitjammer/swift,roambotics/swift,jopamer/swift,arvedviehweger/swift,JaSpa/swift,rudkx/swift,karwa/swift,felix91gr/swift,jtbandes/swift,therealbnut/swift,codestergit/swift,huonw/swift,austinzheng/swift,jmgc/swift,tkremenek/swift,kstaring/swift,return/swift,karwa/swift,roambotics/swift,danielmartin/swift,austinzheng/swift,karwa/swift,danielmartin/swift,stephentyrone/swift,alblue/swift,atrick/swift,shajrawi/swift,jckarter/swift,jckarter/swift,tardieu/swift,shahmishal/swift,xedin/swift,xwu/swift,JaSpa/swift,lorentey/swift,felix91gr/swift,practicalswift/swift,roambotics/swift,modocache/swift,benlangmuir/swift,jopamer/swift,arvedviehweger/swift,arvedviehweger/swift,brentdax/swift,ahoppen/swift,uasys/swift,practicalswift/swift,brentdax/swift,devincoughlin/swift,IngmarStein/swift,gribozavr/swift,ben-ng/swift,jtbandes/swift,kperryua/swift,sschiau/swift,parkera/swift,tjw/swift,danielmartin/swift,hughbe/swift,allevato/swift,danielmartin/swift,kperryua/swift,stephentyrone/swift,tardieu/swift,gribozavr/swift,gottesmm/swift,tjw/swift,shajrawi/swift,sschiau/swift,tjw/swift,shajrawi/swift,therealbnut/swift,huonw/swift,milseman/swift,karwa/swift,swiftix/swift,huonw/swift,parkera/swift,Jnosh/swift,frootloops/swift,devincoughlin/swift,jckarter/swift,kstaring/swift,natecook1000/swift,arvedviehweger/swift,kstaring/swift,CodaFi/swift,therealbnut/swift,deyton/swift,therealbnut/swift,huonw/swift,hooman/swift,benlangmuir/swift,devincoughlin/swift,airspeedswift/swift,airspeedswift/swift,modocache/swift,manavgabhawala/swift,roambotics/swift,uasys/swift,nathawes/swift,milseman/swift,devincoughlin/swift,kperryua/swift,jmgc/swift,felix91gr/swift,airspeedswift/swift,djwbrown/swift,shahmishal/swift,jtbandes/swift,frootloops/swift,sschiau/swift,harlanhaskins/swift,modocache/swift,glessard/swift,devincoughlin/swift,danielmartin/swift,xedin/swift,allevato/swift,deyton/swift,OscarSwanros/swift,parkera/swift,frootloops/swift,rudkx/swift,amraboelela/swift,return/swift,amraboelela/swift,atrick/swift,CodaFi/swift,glessard/swift,calebd/swift,manavgabhawala/swift,hooman/swift,CodaFi/swift,xedin/swift,atrick/swift,harlanhaskins/swift,practicalswift/swift,allevato/swift,JGiola/swift,natecook1000/swift,swiftix/swift,bitjammer/swift,codestergit/swift,parkera/swift,manavgabhawala/swift,xedin/swift,brentdax/swift,alblue/swift,tjw/swift,modocache/swift,jtbandes/swift,jmgc/swift,uasys/swift,tkremenek/swift,shahmishal/swift,apple/swift,roambotics/swift,zisko/swift,ahoppen/swift,calebd/swift,tardieu/swift,gregomni/swift,deyton/swift,CodaFi/swift,aschwaighofer/swift,hughbe/swift,alblue/swift,natecook1000/swift,xwu/swift,jmgc/swift,lorentey/swift,shajrawi/swift,codestergit/swift,hooman/swift,austinzheng/swift,arvedviehweger/swift,brentdax/swift,parkera/swift,jtbandes/swift,zisko/swift,nathawes/swift,glessard/swift,devincoughlin/swift,glessard/swift,karwa/swift,kstaring/swift,shahmishal/swift,stephentyrone/swift,sschiau/swift,rudkx/swift,apple/swift,JaSpa/swift,shajrawi/swift,natecook1000/swift,lorentey/swift,aschwaighofer/swift,hooman/swift,IngmarStein/swift,manavgabhawala/swift,calebd/swift,modocache/swift,return/swift,tardieu/swift,ben-ng/swift,gmilos/swift,frootloops/swift,gmilos/swift,JGiola/swift,airspeedswift/swift,manavgabhawala/swift,shahmishal/swift,gribozavr/swift,bitjammer/swift,Jnosh/swift,kperryua/swift,jmgc/swift,zisko/swift,austinzheng/swift,calebd/swift,manavgabhawala/swift,alblue/swift,CodaFi/swift,ben-ng/swift,amraboelela/swift,rudkx/swift,practicalswift/swift,OscarSwanros/swift,tinysun212/swift-windows,gregomni/swift,gmilos/swift,djwbrown/swift,hughbe/swift,milseman/swift,CodaFi/swift,jmgc/swift,austinzheng/swift,JaSpa/swift,codestergit/swift,tkremenek/swift,apple/swift,tkremenek/swift,benlangmuir/swift,xedin/swift,hooman/swift,ahoppen/swift,felix91gr/swift,alblue/swift,djwbrown/swift,gottesmm/swift,natecook1000/swift,calebd/swift,OscarSwanros/swift,ben-ng/swift,uasys/swift,gregomni/swift,kperryua/swift,natecook1000/swift,gribozavr/swift,codestergit/swift,airspeedswift/swift,jckarter/swift,tardieu/swift,rudkx/swift,deyton/swift,devincoughlin/swift,modocache/swift,gregomni/swift,lorentey/swift,bitjammer/swift,xwu/swift,nathawes/swift,swiftix/swift,ben-ng/swift,harlanhaskins/swift,deyton/swift,OscarSwanros/swift,harlanhaskins/swift,return/swift,OscarSwanros/swift,tinysun212/swift-windows,tkremenek/swift,huonw/swift,karwa/swift,parkera/swift,atrick/swift,roambotics/swift,Jnosh/swift,jopamer/swift,stephentyrone/swift,aschwaighofer/swift,zisko/swift,codestergit/swift,stephentyrone/swift,Jnosh/swift,gmilos/swift,ben-ng/swift,kperryua/swift,JaSpa/swift,apple/swift,arvedviehweger/swift,therealbnut/swift,harlanhaskins/swift,zisko/swift,tjw/swift,return/swift,OscarSwanros/swift,gottesmm/swift,sschiau/swift,jmgc/swift,ahoppen/swift,xwu/swift,gribozavr/swift,therealbnut/swift,benlangmuir/swift,uasys/swift,airspeedswift/swift,swiftix/swift,JGiola/swift,shahmishal/swift,swiftix/swift,tinysun212/swift-windows,glessard/swift,shahmishal/swift,natecook1000/swift,allevato/swift,practicalswift/swift,austinzheng/swift,gmilos/swift,calebd/swift,sschiau/swift,codestergit/swift,frootloops/swift,xedin/swift,alblue/swift,kstaring/swift,amraboelela/swift,jopamer/swift,aschwaighofer/swift,IngmarStein/swift,IngmarStein/swift,harlanhaskins/swift,jckarter/swift,amraboelela/swift,ahoppen/swift,milseman/swift,bitjammer/swift,IngmarStein/swift,IngmarStein/swift,brentdax/swift,jckarter/swift,lorentey/swift,alblue/swift,hooman/swift,shajrawi/swift,practicalswift/swift,glessard/swift,gottesmm/swift,gribozavr/swift,hooman/swift,lorentey/swift,gottesmm/swift,jopamer/swift,stephentyrone/swift,swiftix/swift,djwbrown/swift,tinysun212/swift-windows,calebd/swift,tardieu/swift,swiftix/swift,tardieu/swift,benlangmuir/swift,CodaFi/swift,tjw/swift,xwu/swift,milseman/swift,return/swift,JGiola/swift,lorentey/swift,xwu/swift,allevato/swift,shajrawi/swift,bitjammer/swift,practicalswift/swift,aschwaighofer/swift,atrick/swift,huonw/swift,Jnosh/swift,JGiola/swift,Jnosh/swift,djwbrown/swift,atrick/swift,nathawes/swift,felix91gr/swift,jckarter/swift,ahoppen/swift,arvedviehweger/swift,harlanhaskins/swift,devincoughlin/swift,gottesmm/swift,xwu/swift,tinysun212/swift-windows,danielmartin/swift,nathawes/swift,frootloops/swift,frootloops/swift,tjw/swift,IngmarStein/swift,aschwaighofer/swift,sschiau/swift,austinzheng/swift,huonw/swift,gregomni/swift,allevato/swift,sschiau/swift,amraboelela/swift,gmilos/swift,deyton/swift,parkera/swift,Jnosh/swift,benlangmuir/swift,manavgabhawala/swift,tkremenek/swift,gribozavr/swift,amraboelela/swift,tinysun212/swift-windows,felix91gr/swift,xedin/swift,gottesmm/swift,hughbe/swift,practicalswift/swift,milseman/swift,apple/swift,jtbandes/swift,apple/swift,jtbandes/swift,deyton/swift,parkera/swift,danielmartin/swift,aschwaighofer/swift,return/swift,gmilos/swift,tinysun212/swift-windows,hughbe/swift,brentdax/swift,modocache/swift