MDEV-19845: Make my_cpu.h self-contained

Fix up commit f5c080c7

include/my_cpu.h

 #ifndef MY_CPU_INCLUDED
 #define MY_CPU_INCLUDED
-/* Copyright (c) 2013, MariaDB foundation Ab and SkySQL
+/* Copyright (c) 2013, 2020, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -50,6 +50,13 @@
 # define HAVE_PAUSE_INSTRUCTION /* added in Intel Pentium 4 */
 #endif
 
+#ifdef _WIN32
+#elif defined HAVE_PAUSE_INSTRUCTION
+#elif defined(_ARCH_PWR8)
+#else
+# include "my_atomic.h"
+#endif
+
 static inline void MY_RELAX_CPU(void)
 {
 #ifdef _WIN32

mysys/my_cpu.c

-/* Copyright (c) 2019, MariaDB Corporation.
+/* Copyright (c) 2019, 2020, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -14,7 +14,6 @@
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
 #include <my_global.h>
-#include <my_atomic.h>
 #include <my_cpu.h>
 #include <my_rdtsc.h>
 

storage/innobase/include/ib0mutex.h

 /*****************************************************************************
 
 Copyright (c) 2013, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2019, MariaDB Corporation.
+Copyright (c) 2017, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -29,7 +29,6 @@ Created 2013-03-26 Sunny Bains.
 #ifndef ib0mutex_h
 #define ib0mutex_h
 
-#include "my_atomic.h"
 #include "my_cpu.h"
 #include "os0event.h"
 #include "sync0arr.h"