cuDNN: Help dependents find libs/includes on Power arch (#16128)

On Power architectures cuDNN will install in a target directory. This
sets cuDNN_ROOT to point to the subdirectory to help dependents use
this install.

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

@@ -190,6 +190,11 @@ def url_for_version(self, version):
 
         return url.format(directory, cuda, ver)
 
+    def setup_run_environment(self, env):
+        if 'target=ppc64le: platform=linux' in self.spec:
+            env.set('cuDNN_ROOT', os.path.join(
+                self.prefix, 'targets', 'ppc64le-linux'))
+
     def install(self, spec, prefix):
         install_tree('.', prefix)