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/atmega1280.json

@@ -4,8 +4,7 @@
     "build-tags": ["atmega1280", "atmega"],
     "serial": "uart",
     "ldflags": [
-        "-mmcu=avr51",
-        "-Wl,--defsym=_stack_size=512"
+        "--defsym=_stack_size=512"
     ],
     "linkerscript": "src/device/avr/atmega1280.ld",
     "extra-files": [