Disable atomics on avr target.
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.

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