sh: Set the cfa_offset to 0 if we see a DW_CFA_def_cfa_register op

The way that the CFA is calculated can change as we progress through a
function. If we see a DW_CFA_def_cfa_register op we need to reset the
frame's cfa_offset value which may have been previously setup.

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