avr-*: split avr-gcc-libc into separate packages

- avr-gcc 5.3.0 -> 5.4.0

closes #28220

Since the packages do not share a common prefix anymore, you need
to define the current store paths in your project's Makefile.

Example for an atmega644 build:

CFLAGS += -I /nix/store/9rffxzds5crcpm76g3nr03jx0aa657cf-avr-libc-2.0.0/avr/include
CFLAGS += -B /nix/store/9rffxzds5crcpm76g3nr03jx0aa657cf-avr-libc-2.0.0/avr/lib/avr5
CFLAGS += -L /nix/store/9rffxzds5crcpm76g3nr03jx0aa657cf-avr-libc-2.0.0/avr/lib/avr5
CFLAGS += -L /nix/store/8409dj9js4i5901i63275wxdm783l0p6-avr-gcc-5.4.0/lib/gcc/avr/5.4.0/avr5