[Apple Silicon] [build-script] Build compiler-rt even when cross-compiling
[Apple Silicon] [build-script] Build compiler-rt even when cross-compiling

clang and compiler-rt are rev-locked and should match.  Currently, we build clang (and use it to bootstrap the rest) but don't build compiler-rt.  We should.

This doesn't completely achieve that: this only makes us build compiler-rt for the local host.

This used to be pretty much impossible, but, since then, compiler-rt cmake support was rewritten.

We should build it for the other hosts as well, but that's trickier, because we'd need to make the cross-compiled compiler-rt build use a separate just-built clang (from the local host). We do that for the other projects, but compiler-rt is special (because it's cmake'd as a runtime part of clang/llvm).

atrick/swift,airspeedswift/swift,atrick/swift,allevato/swift,atrick/swift,xwu/swift,airspeedswift/swift,roambotics/swift,apple/swift,rudkx/swift,jckarter/swift,parkera/swift,glessard/swift,glessard/swift,CodaFi/swift,hooman/swift,JGiola/swift,jmgc/swift,tkremenek/swift,atrick/swift,gregomni/swift,xwu/swift,benlangmuir/swift,CodaFi/swift,jckarter/swift,parkera/swift,hooman/swift,CodaFi/swift,roambotics/swift,nathawes/swift,gregomni/swift,benlangmuir/swift,jckarter/swift,benlangmuir/swift,hooman/swift,jmgc/swift,CodaFi/swift,nathawes/swift,xwu/swift,xwu/swift,JGiola/swift,apple/swift,gregomni/swift,hooman/swift,tkremenek/swift,JGiola/swift,parkera/swift,nathawes/swift,tkremenek/swift,allevato/swift,airspeedswift/swift,CodaFi/swift,atrick/swift,rudkx/swift,rudkx/swift,nathawes/swift,apple/swift,gregomni/swift,tkremenek/swift,ahoppen/swift,gregomni/swift,ahoppen/swift,airspeedswift/swift,gregomni/swift,hooman/swift,CodaFi/swift,allevato/swift,glessard/swift,xwu/swift,jckarter/swift,airspeedswift/swift,glessard/swift,parkera/swift,JGiola/swift,apple/swift,nathawes/swift,airspeedswift/swift,ahoppen/swift,jmgc/swift,allevato/swift,jmgc/swift,parkera/swift,JGiola/swift,JGiola/swift,roambotics/swift,allevato/swift,rudkx/swift,benlangmuir/swift,hooman/swift,tkremenek/swift,ahoppen/swift,glessard/swift,hooman/swift,rudkx/swift,CodaFi/swift,atrick/swift,parkera/swift,jckarter/swift,apple/swift,roambotics/swift,nathawes/swift,parkera/swift,glessard/swift,parkera/swift,apple/swift,tkremenek/swift,jmgc/swift,ahoppen/swift,airspeedswift/swift,nathawes/swift,jckarter/swift,allevato/swift,tkremenek/swift,rudkx/swift,roambotics/swift,allevato/swift,jmgc/swift,xwu/swift,roambotics/swift,benlangmuir/swift,ahoppen/swift,jmgc/swift,xwu/swift,jckarter/swift,benlangmuir/swift