Rollup merge of #76364 - fusion-engineering-forks:avr-no-atomic, r=jonas-schievink
Rollup merge of #76364 - fusion-engineering-forks:avr-no-atomic, r=jonas-schievink

Disable atomics on avr target.

`max_atomic_width` was missing in the spec, which means it fell back to the pointer width of 16 bits.

Fixes #76363.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust