Ensure the Host wait4 thread is able to show pid in thread name.

For hosts that cannot support long thread names, shrink the host
wait4 thread name so that it does not truncate the PID in the thread name.

Change by Shawn Best.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216686 91177308-0d34-0410-b5e6-96231b3b80d8