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.

rabit/include/rabit/internal/io.h

@@ -19,8 +19,7 @@
 #include "rabit/internal/utils.h"
 #include "rabit/serializable.h"
 
-namespace rabit {
-namespace utils {
+namespace rabit::utils {
 /*! \brief re-use definition of dmlc::SeekStream */
 using SeekStream = dmlc::SeekStream;
 /**
@@ -31,9 +30,6 @@ struct MemoryFixSizeBuffer : public SeekStream {
   // similar to SEEK_END in libc
   static std::size_t constexpr kSeekEnd = std::numeric_limits<std::size_t>::max();
 
- protected:
-  MemoryFixSizeBuffer() = default;
-
  public:
   /**
    * @brief Ctor
@@ -68,7 +64,7 @@ struct MemoryFixSizeBuffer : public SeekStream {
    * @brief Current position in the buffer (stream).
    */
   std::size_t Tell() override { return curr_ptr_; }
-  virtual bool AtEnd() const { return curr_ptr_ == buffer_size_; }
+  [[nodiscard]] virtual bool AtEnd() const { return curr_ptr_ == buffer_size_; }
 
  protected:
   /*! \brief in memory buffer */
@@ -119,6 +115,5 @@ struct MemoryBufferStream : public SeekStream {
   /*! \brief current pointer */
   size_t curr_ptr_;
 };  // class MemoryBufferStream
-}  // namespace utils
-}  // namespace rabit
+}  // namespace rabit::utils
 #endif  // RABIT_INTERNAL_IO_H_