sh64: oprofile: Fix up kernel stack pointer size mismatch.

For the backtrace code its assumed that the stack pointer is 32-bits,
which is not the case with the sh64 registers. Use the shared
kernel_stack_pointer() helper to get at the actual register, which
already takes care of the necessary typecasting.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>