[gn] Rebase paths in symlink_or_copy against root_build_dir

[phosek]

Summary

We should be always rebasing paths against root_build_dir which is
the directory where scripts are run from, not root_out_dir which is
the current toolchain directory. The latter can result in invalid
paths when the action is being used from a non-default toolchain.

llvm/trunk/utils/gn/build/symlink_or_copy.gni

action(target_name) {
     stamp =
         "$target_gen_dir/" + get_path_info(invoker.output, "file") + ".stamp"
     outputs = [
       stamp,
     ]
     script = "//llvm/utils/gn/build/symlink_or_copy.py"
     args = [
       "--stamp",
-      rebase_path(stamp, root_out_dir),
+      rebase_path(stamp, root_build_dir),
       invoker.source,
-      rebase_path(invoker.output, root_out_dir),
+      rebase_path(invoker.output, root_build_dir),
     ]
   }
 }