AVR Applications using the embedded-nal crate fail to compile

[mutantbob]

If you create an AVR project using cargo generate --git https://github.com/Rahix/avr-hal-template.git and add a dependency on embedded-nal (v 0.6.0) compilation triggers an error

error: cannot find macro `llvm_asm` in this scope
   --> /home/thoth/.cargo/registry/src/github.com-1ecc6299db9ec823/critical-section-0.2.7/src/lib.rs:127:13
    |
127 |             llvm_asm!(
    |             ^^^^^^^^

This is because of a dependency chain
heapless ^0.7 -> atomic-polyfill 0.1.8 -> critical-section 0.2.7

When heapless resolves rust-embedded/heapless#312 it should be possible for embedded-nal to upgrade its dependency and work on modern Rust AVR.