Fix stepper/planner block handling, race conditions (#11098)

- Allow planner to alter the deceleration phase of the currently executing block.
- Remove BUSY flag, as it is NON ATOMIC to set bits in the Stepper ISR and Planner at the same time.

Marlin/src/module/stepper.h

@@ -234,8 +234,6 @@ class Stepper {
 
   public:
 
-    static block_t* current_block;  // A pointer to the block currently being traced
-
     #if ENABLED(X_DUAL_ENDSTOPS) || ENABLED(Y_DUAL_ENDSTOPS) || ENABLED(Z_DUAL_ENDSTOPS)
       static bool homing_dual_axis;
     #endif
@@ -249,6 +247,8 @@ class Stepper {
 
   private:
 
+    static block_t* current_block;          // A pointer to the block currently being traced
+
     static uint8_t last_direction_bits,     // The next stepping-bits to be output
                    axis_did_move;           // Last Movement in the given direction is not null, as computed when the last movement was fetched from planner
 
@@ -360,6 +360,9 @@ class Stepper {
       static uint32_t advance_isr();
     #endif
 
+    // Check if the given block is busy or not - Must not be called from ISR contexts
+    static bool is_block_busy(const block_t* const block);
+
     // Get the position of a stepper, in steps
     static int32_t position(const AxisEnum axis);