[XR] Add a linker file to preserve the ARCore and ARKit assemblies when using IL2CPP

Assets/link.xml

+<linker>
+    <assembly fullname="Unity.XR.ARCore" preserve="all"/>
+    <assembly fullname="Unity.XR.ARKit" preserve="all"/>
+</linker>

Assets/link.xml.meta

+fileFormatVersion: 2
+guid: ee957744b1fd90c429ee647899f73f60
+TextScriptImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: