sh: Provide diagnostic kernel stack checks

Enable kernel stack checking code in both the dynamic ftrace and mcount
code paths. Check the stack to see if it's overflowing and make sure
that the stack pointer contains an address that's either in init_stack
or after the bss.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>