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

.github/workflows/linux.yml

@@ -133,11 +133,6 @@ jobs:
           mkdir -p ~/lib
           tar -C ~/lib -xf tinygo.linux-amd64.tar.gz
           ln -s ~/lib/tinygo/bin/tinygo ~/go/bin/tinygo
-      - name: Install apt dependencies
-        run: |
-          sudo apt-get install --no-install-recommends \
-              gcc-avr \
-              avr-libc
       - run: make tinygo-test-wasi-fast
       - run: make smoketest
   assert-test-linux:
@@ -158,8 +153,6 @@ jobs:
               qemu-system-arm \
               qemu-system-riscv32 \
               qemu-user \
-              gcc-avr \
-              avr-libc \
               simavr \
               ninja-build
       - name: Install Go

.github/workflows/windows.yml

@@ -125,7 +125,7 @@ jobs:
         run: 7z x release.zip -r
       - name: Smoke tests
         shell: bash
-        run: make smoketest TINYGO=$(PWD)/build/tinygo/bin/tinygo AVR=0
+        run: make smoketest TINYGO=$(PWD)/build/tinygo/bin/tinygo
 
   stdlib-test-windows:
     runs-on: windows-2022