tile: set up initial stack top to honor STACK_TOP_DELTA

For some reason this was never changed to match the rest of the
code where we always initialize the kernel sp 64 bytes below
the top of the page.  This is generally harmless, but it does
mean that if you do a dump_stack() early on in kernel boot you
see a bogus warning about stack overrun.

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