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

[Anton Korobeynikov via llvm-commits]

asl wrote:

@jyknight I believe MSP430 should be pretty similar with AVR here. But let me think a bit about how the atomics should be structured there (I have a long-standing TODO in my list for this)

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