powerpc: Fix _CPU_Counter_read() for MPC860

The mpc860 is a RTEMS-specific multilib define, see GCC
"gcc/config/rs6000/rtems.h".

cpukit/score/cpu/powerpc/include/rtems/score/cpu.h

@@ -745,6 +745,8 @@ static inline CPU_Counter_ticks _CPU_Counter_read( void )
 #if defined(__PPC_CPU_E6500__)
   /* Use Alternate Time Base */
   __asm__ volatile( "mfspr %0, 526" : "=r" (value) );
+#elif defined(mpc860)
+  __asm__ volatile( "mftb %0" : "=r" (value) );
 #else
   __asm__ volatile( "mfspr %0, 268" : "=r" (value) );
 #endif