sh: Replace DEBUG_STACKOVERFLOW with STACK_DEBUG.

STACK_DEBUG ties in to mcount in order to do function-granular stack
overflow checks as opposed to lazily checking from IRQ context. As the
default is nohz, the frequency of overflow checking is too irregular to
catch much useful information, and so the mcount approach employed by
sparc64 is adopted instead.

This kills off the old check entirely from the do_IRQ() path and now
adopts CONFIG_MCOUNT instead.

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