[3.x] libvlc: use thread_local instead of a work-around in pthread

The previously used workaround that was needed for iOS 8 and the 32bit Intel iOS simulator caused low level and hard to reproduce threading issues.

This MR restricts the use of the work-around to the 32bit Intel simulator and deploys thread_local everywhere else.

This raises the deployment target from iOS 8.4 to 9.0 matching the official VLC-iOS version.