avr: use new compilation infrastructure

Gets rid of:
  avrbinutils
  avrgcc

to replace with:
  pkgsCross.avr.buildPackages.binutils
  pkgsCross.avr.buildPackages.gcc

lib/systems/inspect.nix

@@ -19,6 +19,7 @@ rec {
     isRiscV        = { cpu = { family = "riscv"; }; };
     isSparc        = { cpu = { family = "sparc"; }; };
     isWasm         = { cpu = { family = "wasm"; }; };
+    isAvr          = { cpu = { family = "avr"; }; };
 
     is32bit        = { cpu = { bits = 32; }; };
     is64bit        = { cpu = { bits = 64; }; };