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.

.circleci/config.yml

@@ -63,8 +63,6 @@ commands:
                 clang-<<parameters.llvm>> \
                 libclang-<<parameters.llvm>>-dev \
                 lld-<<parameters.llvm>> \
-                gcc-avr \
-                avr-libc \
                 cmake \
                 ninja-build
       - hack-ninja-jobs