Use gettid() on Linux.

Use gettid() on Linux.

This is a hack for two reasons:

  o  Current glibc releases (up to 2.5) have no wrapper for gettid(),
     so we have to use _syscall0 to access it.
  o  There is no guarantee that pthreads will correspond to kernel
     threads. However, in the two existing implementations of pthreads
     on Linux, they correspond either to kernel processes or kernel
     threads, so assumptions that the (pid, tid) pair is unique are
     correct.

On the other hand, using real thread ID's has many advantages,
so I am keeping this patch at least in the dev-sorter branch to see
if it's safe or not.