arc32: platform.txt: sketch_flags, -DPLATFORM={version}

[phd]

platform.txt:

• sketch_flags for use with ./platform.sketch.txt
• define -DPLATFORM= with platform {version}
• added -DARDUINO_ARCH= and -DARDUINO_BOARD=

See: https://github.com/arduino/arduino-builder/pull/282

[ryantm]

https://github.com/arduino/ArduinoCore-arc32/pull/600 might be relevant to this. perhaps sketch_flags is not needed if extra_flags is left for the user to use.

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