OSSpinLock is unsafe in iOS

[vampirewalk]

Hi ibireme, thank you for bring us this great library.
My concern is that OSSpinLock is unsafe in iOS. It does not guarantee progress in the face of priority inversion.

http://mjtsai.com/blog/2015/12/16/osspinlock-is-unsafe/
https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html

[ibireme]

Thanks very much!
I'll do some research and fix it later.

[ibireme]

Fixed