Rollup merge of #114495 - taiki-e:avr-atomic, r=Amanieu

Set max_atomic_width for AVR to 16

This is currently set to 0

https://github.com/rust-lang/rust/blob/90f0b24ad3e7fc0dc0e419c9da30d74629cd5736/compiler/rustc_target/src/spec/avr_gnu_base.rs#L26-L27

However, LLVM supports {8,16}-bit atomic load/store on AVR (support for RMW is still quite incomplete and only partially supported).

https://github.com/llvm/llvm-project/blob/llvmorg-15.0.0/llvm/test/CodeGen/AVR/atomics/load8.ll#L5-L13
https://github.com/llvm/llvm-project/blob/llvmorg-15.0.0/llvm/test/CodeGen/AVR/atomics/load16.ll#L3-L12
https://github.com/llvm/llvm-project/blob/llvmorg-15.0.0/llvm/test/CodeGen/AVR/atomics/store.ll#L3-L22

cc #99668

r? `@Amanieu`

Author: matthiaskrgr