sh: oprofile: Backtrace support.

This patch improves the oprofile support on sh and adds backtrace
support.

Signed-off-by: Dave Peverley <dpeverley@mpc-data.co.uk>
Signed-off-by: Chris Smith <chris.smith@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>