Add a C_AXIS and Z_HEAD to support COREXZ

Marlin/Marlin.h

  * A_AXIS and B_AXIS are used by COREXY printers
  * X_HEAD and Y_HEAD is used for systems that don't have a 1:1 relationship between X_AXIS and X Head movement, like CoreXY bots.
  */
-enum AxisEnum {X_AXIS=0, Y_AXIS=1, A_AXIS=0, B_AXIS=1, Z_AXIS=2, E_AXIS=3, X_HEAD=4, Y_HEAD=5};
+enum AxisEnum {X_AXIS=0, A_AXIS=0, Y_AXIS=1, B_AXIS=1, Z_AXIS=2, C_AXIS=2, E_AXIS=3, X_HEAD=4, Y_HEAD=5, Z_HEAD=5};
 
 enum EndstopEnum {X_MIN=0, Y_MIN=1, Z_MIN=2, Z_PROBE=3, X_MAX=4, Y_MAX=5, Z_MAX=6, Z2_MIN=7, Z2_MAX=8};