Restore LF_BACKOFF

Moved InnoDB UT_RELAX_CPU() to server. Restored cross-platform LF_BACKOFF
implementation basing on UT_RELAX_CPU().

include/my_cpu.h

@@ -1,3 +1,5 @@
+#ifndef MY_CPU_INCLUDED
+#define MY_CPU_INCLUDED
 /* Copyright (c) 2013, MariaDB foundation Ab and SkySQL
 
    This program is free software; you can redistribute it and/or modify
@@ -42,3 +44,58 @@
 #define HMT_medium_high()
 #define HMT_high()
 #endif
+
+
+static inline void MY_RELAX_CPU(void)
+{
+#ifdef HAVE_PAUSE_INSTRUCTION
+  /*
+    According to the gcc info page, asm volatile means that the
+    instruction has important side-effects and must not be removed.
+    Also asm volatile may trigger a memory barrier (spilling all registers
+    to memory).
+  */
+#ifdef __SUNPRO_CC
+  asm ("pause" );
+#else
+  __asm__ __volatile__ ("pause");
+#endif
+
+#elif defined(HAVE_FAKE_PAUSE_INSTRUCTION)
+  __asm__ __volatile__ ("rep; nop");
+#elif defined _WIN32
+  /*
+    In the Win32 API, the x86 PAUSE instruction is executed by calling
+    the YieldProcessor macro defined in WinNT.h. It is a CPU architecture-
+    independent way by using YieldProcessor.
+  */
+  YieldProcessor();
+#elif defined(_ARCH_PWR8)
+  __ppc_get_timebase();
+#else
+  int32 var, oldval = 0;
+  my_atomic_cas32_strong_explicit(&var, &oldval, 1, MY_MEMORY_ORDER_RELAXED,
+                                  MY_MEMORY_ORDER_RELAXED);
+#endif
+}
+
+
+/*
+  LF_BACKOFF should be used to improve performance on hyperthreaded CPUs. Intel
+  recommends to use it in spin loops also on non-HT machines to reduce power
+  consumption (see e.g http://softwarecommunity.intel.com/articles/eng/2004.htm)
+
+  Running benchmarks for spinlocks implemented with InterlockedCompareExchange
+  and YieldProcessor shows that much better performance is achieved by calling
+  YieldProcessor in a loop - that is, yielding longer. On Intel boxes setting
+  loop count in the range 200-300 brought best results.
+*/
+
+static inline int LF_BACKOFF(void)
+{
+  int i;
+  for (i= 0; i < 200; i++)
+    MY_RELAX_CPU();
+  return 1;
+}
+#endif