Adjust workaround for #41 once Xcode 13.3 is out

[lorentey]

Beta builds of Xcode 13.3 fix the double wide atomic load issue in https://github.com/apple/swift-atomics/issues/41, so the workaround is no longer necessary to apply in Swift 5.6.

Once Xcode 13.3 gets released, adjust the code to only apply the workaround in Swift 5.5. (Or bump the minimum Swift toolchain requirement to 5.6, and remove it altogether.)