Report original thread ID for FreeBSD core files
On FreeBSD the tid is (somewhat unintuitively) found in the pr_pid
field of the NT_PRSTATUS note. Collect it when parsing the note and
store it in the thread data.
For Linux I've left the original behaviour of using sequential TIDs
(0, 1, 2...) as I don't yet have code to obtain it.
Differential Revision: http://reviews.llvm.org/D11652