Fix __ARM__ / __arm__ typo (#19063)

Marlin/src/module/stepper.cpp

     }
 
     FORCE_INLINE int32_t Stepper::_eval_bezier_curve(const uint32_t curr_step) {
-      #if defined(__ARM__) || defined(__thumb__)
+      #if defined(__arm__) || defined(__thumb__)
 
         // For ARM Cortex M3/M4 CPUs, we have the optimized assembler version, that takes 43 cycles to execute
         uint32_t flo = 0;