Fixup The Cross Build of SwiftSyntax
Fixup The Cross Build of SwiftSyntax

Propagate DCMAKE_Swift_COMPILER_TARGET with the host architecture of the
variant we're trying to build so CMake will forward the correct -arch
- which is to say, the triple of the host we're trying to build against,
not the machine that it's building on currently.

Resolves rdar://80383591, SR-14904

parkera/swift,ahoppen/swift,glessard/swift,hooman/swift,atrick/swift,roambotics/swift,JGiola/swift,gregomni/swift,JGiola/swift,atrick/swift,JGiola/swift,atrick/swift,xwu/swift,tkremenek/swift,rudkx/swift,benlangmuir/swift,ahoppen/swift,parkera/swift,JGiola/swift,xwu/swift,tkremenek/swift,xwu/swift,benlangmuir/swift,rudkx/swift,ahoppen/swift,apple/swift,xwu/swift,xwu/swift,parkera/swift,gregomni/swift,rudkx/swift,parkera/swift,tkremenek/swift,roambotics/swift,ahoppen/swift,parkera/swift,hooman/swift,ahoppen/swift,roambotics/swift,glessard/swift,roambotics/swift,benlangmuir/swift,apple/swift,hooman/swift,apple/swift,roambotics/swift,parkera/swift,tkremenek/swift,parkera/swift,apple/swift,gregomni/swift,gregomni/swift,tkremenek/swift,ahoppen/swift,JGiola/swift,atrick/swift,hooman/swift,tkremenek/swift,rudkx/swift,glessard/swift,apple/swift,glessard/swift,apple/swift,atrick/swift,hooman/swift,gregomni/swift,glessard/swift,benlangmuir/swift,tkremenek/swift,roambotics/swift,JGiola/swift,hooman/swift,rudkx/swift,benlangmuir/swift,xwu/swift,xwu/swift,atrick/swift,benlangmuir/swift,hooman/swift,parkera/swift,gregomni/swift,glessard/swift,rudkx/swift