MDEV-25807: ARM build failure due to missing ISB instruction on ARMv6

Debian has support for 3 different arm machine and probably one which is
failing is a pretty old version which doesn't support the said instruction.

So we can make it specific to _aarch64_ (as per the arm official reference
manual ISB should be defined by all ARMv8 processors). [as per the ARMv7 specs
even 32 bits processor should support it but not sure which exact version
Debian has under armel].

In either case, the said performance issue will have an impact mainly with a
high-end processors with extreme parallelism so it safe to limit it to _aarch64_.

Corrects: MDEV-24630

include/my_cpu.h

@@ -84,7 +84,15 @@ static inline void MY_RELAX_CPU(void)
   __ppc_get_timebase();
 #elif defined __GNUC__ && (defined __arm__ || defined __aarch64__)
   /* Mainly, prevent the compiler from optimizing away delay loops */
+#ifdef _aarch64_
   __asm__ __volatile__ ("isb":::"memory");
+#else
+  /*
+    some older 32 bits processor doesn't support isb but as per
+    arm-v8 reference manual all armv8 processor should support isb.
+  */
+  __asm__ __volatile__ ("":::"memory");
+#endif
 #else
   int32 var, oldval = 0;
   my_atomic_cas32_strong_explicit(&var, &oldval, 1, MY_MEMORY_ORDER_RELAXED,