tile: fix "odd fault" warning for stack backtraces

We were setting ex1 in new kernel threads to KERNEL_PL.
But since we just do a simple context-switch, not an iret,
any value set here is ignored anyway, and its presence causes
stack backtraces to end with a warning about an "odd fault".

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>