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.

.github/workflows/build-macos.yml

@@ -93,7 +93,7 @@ jobs:
           path: build/tinygo.darwin-amd64.tar.gz
       - name: Smoke tests
         shell: bash
-        run: make smoketest TINYGO=$(PWD)/build/tinygo AVR=0
+        run: make smoketest TINYGO=$(PWD)/build/tinygo
   test-macos-homebrew:
     name: homebrew-install
     runs-on: macos-latest