[FR] Add support for DIRECT_STEPPING with NUM_AXES >=4

[DerAndere1]

Description

Add support for DIRECT_STEPPING with NUM_AXES >=4 to https://github.com/MarlinFirmware/Marlin/tree/bugfix-2.1.x

Steps to Reproduce

1. See stepper.cpp lines 1602 ff., lines 1846 ff., etc.

Expected behavior:
DIRECT_STEPPING working with NUM_AXES >= 4

Actual behavior:
See stepper.cpp lines 1602 ff., 1846 ff. etc.

Additional Information

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