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/digispark.json

@@ -2,8 +2,8 @@
 	"inherits": ["attiny85"],
 	"build-tags": ["digispark"],
 	"ldflags": [
-		"-Wl,--defsym=_bootloader_size=2180",
-		"-Wl,--defsym=_stack_size=128"
+		"--defsym=_bootloader_size=2180",
+		"--defsym=_stack_size=128"
 	],
 	"flash-command": "micronucleus --run {hex}",
 	"emulator": "simavr -m attiny85 -f 16000000 {}"