Use ptr from mmap for GHistIndexMatrix and ColumnMatrix. (#9315)

* Use ptr from mmap for `GHistIndexMatrix` and `ColumnMatrix`.

- Define a resource for holding various types of memory pointers.
- Define ref vector for holding resources.
- Swap the underlying resources for GHist and ColumnM.
- Add documentation for current status.
- s390x support is removed. It should work if you can compile XGBoost, all the old workaround code does is to get GCC to compile.

doc/tutorials/index.rst

@@ -11,22 +11,22 @@ See `Awesome XGBoost <https://github.com/dmlc/xgboost/tree/master/demo>`_ for mo
 
   model
   saving_model
+  learning_to_rank
+  dart
+  monotonic
+  feature_interaction_constraint
+  aft_survival_analysis
+  categorical
+  multioutput
+  rf
   kubernetes
   Distributed XGBoost with XGBoost4J-Spark <https://xgboost.readthedocs.io/en/latest/jvm/xgboost4j_spark_tutorial.html>
   Distributed XGBoost with XGBoost4J-Spark-GPU <https://xgboost.readthedocs.io/en/latest/jvm/xgboost4j_spark_gpu_tutorial.html>
   dask
   spark_estimator
   ray
-  dart
-  monotonic
-  rf
-  feature_interaction_constraint
-  learning_to_rank
-  aft_survival_analysis
+  external_memory
   c_api_tutorial
   input_format
   param_tuning
-  external_memory
   custom_metric_obj
-  categorical
-  multioutput