CUDA: CUDAHOSTCXX Env (#16869)

This is a general CMake CUDA language hint to use the CXX
compiler has host compiler for NVCC. Seems like a good
default since we do not express the CUDA compiler in Spack
otherwise yet (e.g. no `self.compiler.cuda` or
`self.compiler.cudahostcxx`).

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

@@ -77,6 +77,7 @@ class Cuda(Package):
     depends_on('libxml2', when='@10.1.243:')
 
     def setup_build_environment(self, env):
+        env.set('CUDAHOSTCXX', self.compiler.cxx)
         if self.spec.satisfies('@10.1.243:'):
             libxml2_home  = self.spec['libxml2'].prefix
             env.set('LIBXML2HOME', libxml2_home)
@@ -84,6 +85,7 @@ def setup_build_environment(self, env):
 
     def setup_run_environment(self, env):
         env.set('CUDA_HOME', self.prefix)
+        env.set('CUDAHOSTCXX', self.compiler.cxx)
 
     def install(self, spec, prefix):
         if os.path.exists('/tmp/cuda-installer.log'):