arm: Use CPU_TIMESTAMP_USE_STRUCT_TIMESPEC

Converting 64-bit nanoseconds values into the common struct timeval or
struct timespec formats requires a 64-bit division to get the seconds
value.  Performance analysis of high network loads revealed that this is
too costly on ARM.

cpukit/score/cpu/arm/rtems/score/cpu.h

@@ -149,7 +149,7 @@
 /* XXX Why 32? */
 #define CPU_STRUCTURE_ALIGNMENT __attribute__ ((aligned (32)))
 
-#define CPU_TIMESTAMP_USE_INT64_INLINE TRUE
+#define CPU_TIMESTAMP_USE_STRUCT_TIMESPEC TRUE
 
 /*
  * The interrupt mask disables only normal interrupts (IRQ).