sh: Fix up stack overflow check with ftrace disabled.

Presently the STACK_CHECK() code is called in to multiple times, although
it's only necessary from the mcount entry. The code still attempts to
treat the nop case as an ftrace path resulting in superfluous code flow
for the case where ftrace is disabled. And finally, this also fixes up
references to a few undefined symbols when FUNCTION_TRACER=n.

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