Commit a work-in-progress system runtime for Mac OS X which won't
do anything right now.  Add a few new methods to the Thread base
class which HistoryThread needs.  I think I updated all the 
CMakeLists files correctly for the new plugin.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@194756 91177308-0d34-0410-b5e6-96231b3b80d8