Add LED_BUILTIN macro to tiny8

Corresponding to the Tiny Programmer test LED on pin0

Relevant Arduino commits
https://github.com/arduino/Arduino/commit/350eb86dca5c679bd0d4913baae3de0440b599a3
https://github.com/arduino/Arduino/commit/9b519f2fbc85a3f37fde9aacc73a7e8aea299463