aarch64: support UT_RELAX_CPU macro

an isb is a hardware block that produces some stall without using ALU or
other CPU resources. It is fitting for this function.

Author: Tzachi Zidenberg

storage/innobase/include/ut0ut.h

@@ -115,6 +115,8 @@ to memory). */
 the YieldProcessor macro defined in WinNT.h. It is a CPU architecture-
 independent way by using YieldProcessor. */
 #define UT_RELAX_CPU() YieldProcessor()
+#elif defined(__aarch64__)
+#define UT_RELAX_CPU() __asm__ __volatile__("isb" ::: "memory")
 #else
 #define UT_RELAX_CPU() __asm__ __volatile__("" ::: "memory")
 #endif