community/{binutils-cross-embedded,gcc-avr}: upgrade

This MR does the following things:

1. upgrade community/gcc-avr to 10.2.0
2. upgrade community/binutils-cross-embedded 2.35.2
3. change the toolchain paths of the AVR toolchain to /usr/avr/{lib,include} for both binutils and GCC (this has to be done in lock-step)
   • this matches all the other embedded toolchains in Alpine, and also Ubuntu, Arch etc. use /usr/$target_triple/... for embedded toolchains
4. some housekeeping in community/gcc-avr
   • use system ISL
   • fix license identifier
   • compile with --with-avrlibc