Use correct host/target CMake variables to define lit variables.

CMake and autotools disagree on what "host" means in a cross-compilation
context. Autotools (and lit) take it to be the machine the binaries being
compiled now will run on. CMake takes it to be the machine actually compiling
the binaries now.

This change makes lit.site-cfg more consistent between autotools and CMake,
allowing lit tests (particularly in ExecutionEngine) to run correctly when
cross-compiled with CMake