feat(ESP32): Add "-DARDUINO_ESP32_MCU_{build.mcu}" macro to platform.txt file

[kiwi64ajs]

Description of Change

Adding this macro definition allows MCU-specific preprocessor conditional expressions, enabling libraries to handle MCU-specific differences between the ESP32 MCUs and IDF data structures.

The specific issue I could not find a workaround for was handling the difference in UART State Structures which this macro allowed me to write code like the sample below, where the UART State register is located in different location/structure on some ESP32 MCUs.

			uart_dev_t *hw = UART_LL_GET_HW(_uart_nr);		 	

 		#if defined(ARDUINO_ESP32_MCU_esp32c3) || defined(ARDUINO_ESP32_MCU_esp32s3)    
		  	return hw->fsm_status.st_urx_out != 0;
			
 		#elif defined(ARDUINO_ESP32_MCU_esp32)  
 			return hw->status.st_urx_out != 0;

 		#endif

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Warnings

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Some issues found for the commit messages in this PR: the commit message "added -DARDUINO_ESP32_MCU_{build.mcu} ": summary looks empty type/action looks empty Please fix these commit messages - here are some basic tips: follow Conventional Commits style correct format of commit message should be: <type/action>(<scope/component>): <summary>, for example fix(esp32): Fixed startup timeout issue allowed types are: change,ci,docs,feat,fix,refactor,remove,revert,test sufficiently descriptive message summary should be between 20 to 72 characters and start with upper case letter avoid Jira references in commit messages (unavailable/irrelevant for our customers) TIP: Install pre-commit hooks and run this check when committing (uses the Conventional Precommit Linter).

👋 Hello kiwi64ajs, we appreciate your contribution to this project!

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[lucasssvaz]

There is no need to create new macros, you can use the ones already provided by ESP-IDF. Here are some examples:

https://github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp32-hal-uart.c#L1023C5-L1023C28
https://github.com/espressif/arduino-esp32/blob/master/libraries/RainMaker/examples/RMakerSwitch/RMakerSwitch.ino#L12