[llvm] Set MaxAtomicSizeInBitsSupported for remaining targets. (PR #75703)

[James Y Knight via llvm-commits]

jyknight wrote:

In further investigation I realized AVR is actually only partially functional today; the backend is broken if you attempt to use larger sized atomic operations. I now expect to also convert it to use AtomicExpandPass, but with NotAtomic lowering, and custom "fences" for disabling/enabling interrupts.

That'll be future work; I'll continue to leave it untouched here.

https://github.com/llvm/llvm-project/pull/75703