[MDEV-28162] Replace PFS_atomic with std::atomic<T>

PFS_atomic class contains wrappers around my_atomic_* operations, which
are macros to GNU atomic operations (__atomic_*). Due to different
implementations of compilers, clang may encounter errors when compiling
on x86_32 architecture.

The following functions are replaced with C++ std::atomic type in
performance schema code base:
  - PFS_atomic::store_*()
      -> my_atomic_store*
        -> __atomic_store_n()
    => std::atomic<T>::store()

  - PFS_atomic::load_*()
      -> my_atomic_load*
        -> __atomic_load_n()
    => std::atomic<T>::load()

  - PFS_atomic::add_*()
      -> my_atomic_add*
        -> __atomic_fetch_add()
    => std::atomic<T>::fetch_add()

  - PFS_atomic::cas_*()
    -> my_atomic_cas*
      -> __atomic_compare_exchange_n()
    => std::atomic<T>::compare_exchange_strong()

and PFS_atomic class could be dropped completely.

Note that in the wrapper memory order passed to original GNU atomic
extensions are hard-coded as `__ATOMIC_SEQ_CST`, which is equivalent to
`std::memory_order_seq_cst` in C++, and is the default parameter for
std::atomic_* functions.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services.

storage/perfschema/pfs_user.h

@@ -28,6 +28,8 @@
   Performance schema user (declarations).
 */
 
+#include <atomic>
+
 #include "pfs_lock.h"
 #include "lf.h"
 #include "pfs_con_slice.h"
@@ -58,22 +60,22 @@ struct PFS_ALIGNED PFS_user : public PFS_connection_slice
 public:
   inline void init_refcount(void)
   {
-    PFS_atomic::store_32(& m_refcount, 1);
+    m_refcount.store(1);
   }
 
   inline int get_refcount(void)
   {
-    return PFS_atomic::load_32(& m_refcount);
+    return m_refcount.load();
   }
 
   inline void inc_refcount(void)
   {
-    PFS_atomic::add_32(& m_refcount, 1);
+    m_refcount.fetch_add(1);
   }
 
   inline void dec_refcount(void)
   {
-    PFS_atomic::add_32(& m_refcount, -1);
+    m_refcount.fetch_sub(1);
   }
 
   void aggregate(bool alive);
@@ -97,7 +99,7 @@ public:
   ulonglong m_disconnected_count;
 
 private:
-  int m_refcount;
+  std::atomic<int> m_refcount;
 };
 
 int init_user(const PFS_global_param *param);