[llvm] [ARM] Use correct ABI for atomic functions (PR #128891)

[Jon Roelofs via llvm-commits]

jroelofs wrote:

> I noticed that AArch64 is doing this extension in some cases, but that isn't needed because in the AArch64 PCS the high bits are unspecified, the callee does the extension. There was already a target lowering hook to configure this, so I've set it correctly for AArch64

The Darwin ABI is a little different here:

> The caller of a function is responsible for signing or zero-extending any argument with fewer than 32 bits.

https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Pass-arguments-to-functions-correctly

https://github.com/llvm/llvm-project/pull/128891