FindCUDAToolkit: Fix cross-compiling without CMAKE_SYSTEM_PROCESSOR

Previously we relied on the cross-compiling toolchain file to set
`CMAKE_SYSTEM_PROCESSOR` to a value consistent with the CUDA compiler's
target architecture.  When a compiled language is enabled, we now have
its target architecture in `CMAKE_<LANG>_COMPILER_ARCHITECTURE_ID`.
Use that to select the CUDA toolkit's corresponding target directory.

If no language is enabled, or the compiler's target architecture is not
known, fall back to `CMAKE_SYSTEM_PROCESSOR`.  If that is not set, fail
with an explicit error make clear what is wrong.