Pre-calculate diagonal rod length squared

Marlin/Configuration.h

 #define DELTA_TOWER3_X 0.0 // back middle tower
 #define DELTA_TOWER3_Y DELTA_RADIUS
 
+// Diagonal rod squared
+#define DELTA_DIAGONAL_ROD_2 pow(DELTA_DIAGONAL_ROD,2)
+
 //===========================================================================
 //=============================Thermal Settings  ============================
 //===========================================================================

Marlin/Marlin_main.cpp

 #ifdef DELTA
 void calculate_delta(float cartesian[3])
 {
-  delta[X_AXIS] = sqrt(sq(DELTA_DIAGONAL_ROD)
+  delta[X_AXIS] = sqrt(DELTA_DIAGONAL_ROD_2
                        - sq(DELTA_TOWER1_X-cartesian[X_AXIS])
                        - sq(DELTA_TOWER1_Y-cartesian[Y_AXIS])
                        ) + cartesian[Z_AXIS];
-  delta[Y_AXIS] = sqrt(sq(DELTA_DIAGONAL_ROD)
+  delta[Y_AXIS] = sqrt(DELTA_DIAGONAL_ROD_2
                        - sq(DELTA_TOWER2_X-cartesian[X_AXIS])
                        - sq(DELTA_TOWER2_Y-cartesian[Y_AXIS])
                        ) + cartesian[Z_AXIS];
-  delta[Z_AXIS] = sqrt(sq(DELTA_DIAGONAL_ROD)
+  delta[Z_AXIS] = sqrt(DELTA_DIAGONAL_ROD_2
                        - sq(DELTA_TOWER3_X-cartesian[X_AXIS])
                        - sq(DELTA_TOWER3_Y-cartesian[Y_AXIS])
                        ) + cartesian[Z_AXIS];