investigate Mac LDT slot swapping by kernel

From bruen...@google.com on March 28, 2014 21:32:08

• It seems that the kernel does not swap our entries, so we are currently
  • creating separate entries per thread -- but we only need to save the
  • ones the app might use which we assume will be <= 3.

Originally I thought our LDT slot would be swapped. This issue covers investigating further.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1405