elf-core: handle core with a single NT_PRPSINFO (not one per thread)

On FreeBSD we have only one NT_PRPSINFO in a core file, regardless of the
number of threads.  Consider a new thread to start whenever we see another
instance of either NT_PRPSINFO or NT_PRSTATUS.

Thanks to Samuel Jacob for testing this patch on Linux cores.

llvm-svn: 186633