Add variant to allow unsupported compiler & CUDA combinations (#19736)

Sometimes users need to be able to override the conflicts in `CudaPacakge`.  This introduces a variant to enable/disable them.

var/spack/repos/builtin/packages/cuda/package.py

@@ -122,6 +122,8 @@ class Cuda(Package):
     conflicts('arch=darwin-mojave-x86_64')
 
     variant('dev', default=False, description='Enable development dependencies, i.e to use cuda-gdb')
+    variant('allow-unsupported-compilers', default=False,
+            description='Allow unsupported host compiler and CUDA version combinations')
 
     depends_on('libxml2', when='@10.1.243:')
     # cuda-gdb needs libncurses.so.5