TLS/GC issues on macOS

[Sönke Ludwig]

There appears to be a GC visibility issue of TLS sections on macOS when 
multiple threads are involved. I added a test case to what looks like 
the same issue: https://github.com/ldc-developers/ldc/issues/2187

The question is, does anyone have enough insight on the TLS 
implementation to make a quick diagnosis or fix?

This is biting us pretty hard in our upcoming commercial project (DMD 
has codegen issues on macOS), so any help would be highly appreciated.