AVR GPIO macro defines more readable (#5937)

AVR GPIO macro defines more readable (#5937)

* A little easier to read the definition of the GPIO control macro for AVR.

No change in build result.

* Changed to not use GNU statement expression extension.

No change in build result.

* Modified split_common/serial.c to use qmk_firmware standard GPIO control macro.

No change in build result.

* fix PE6 -> E6

* remove some space

* add some comment to config_common.h

* Changed split_common/serial.c to use a newer version of qmk_firmware standard GPIO control macro.