fix: use the platform integer type

Author: ianna

src/awkward/_reducers.py

@@ -67,10 +67,11 @@ def _dtype_for_kernel(cls, dtype: DTypeLike) -> DTypeLike:
     @classmethod
     def _promote_integer_rank(cls, given_dtype: DTypeLike) -> DTypeLike:
         if given_dtype in (np.bool_, np.int8, np.int16, np.int32):
-            return np.int32 if cls._use32 else np.int64
+            # Typically, np.intp is used to represent the platform integer type
+            return np.int32 if cls._use32 else np.intp
 
         elif given_dtype in (np.uint8, np.uint16, np.uint32):
-            return np.uint32 if cls._use32 else np.uint64
+            return np.uint32 if cls._use32 else np.intp
 
         else:
             return given_dtype