sh: initial stack protector support.

This implements basic -fstack-protector support, based on the early ARM
version in c743f380. The SMP case is
limited to the initial canary value, while the UP case handles per-task
granularity (limited to 32-bit sh until a new enough sh64 compiler
manifests itself).

Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Reviewed-by: Carmelo Amoroso <carmelo.amoroso@st.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>