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/atmega328p.json

@@ -3,9 +3,6 @@
 	"cpu": "atmega328p",
 	"build-tags": ["atmega328p", "atmega", "avr5"],
 	"serial": "uart",
-	"ldflags": [
-		"-mmcu=avr5"
-	],
 	"linkerscript": "src/device/avr/atmega328p.ld",
 	"extra-files": [
 		"targets/avr.S",