[POWERPC] spufs: Add tid file

The new tid file contains the ID of the thread currently running the
context, if any.  This is used so that the new spu-top and spu-ps
tools can find the thread in /proc.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>