avr: drop GNU toolchain dependency

- Use compiler-rt and picolibc instead of avr-libc.
  - Use ld.lld instead of avr-ld (or avr-gcc).

This makes it much easier to get started with TinyGo on AVR because
installing these extra tools (gcc-avr, avr-libc) can be a hassle.
It also opens the door for future improvements such as ThinLTO.

There is a code size increase but I think it's worth it in the long run.
The code size increase can hopefully be reduced with improvements to the
LLVM AVR backend and to compiler-rt.

targets/arduino-mega2560.json

@@ -2,7 +2,7 @@
     "inherits": ["atmega2560"],
     "build-tags": ["arduino_mega2560"],
     "ldflags": [
-        "-Wl,--defsym=_bootloader_size=8192"
+        "--defsym=_bootloader_size=8192"
     ],
     "flash-command":"avrdude -c wiring -b 115200 -p atmega2560 -P {port} -U flash:w:{hex}:i -v -D"
 }