sh: unwinder: Set the flags for DW_CFA_val_offset ops as DWARF_VAL_OFFSET

The handling of DW_CFA_val_offset ops was incorrectly using the
DWARF_REG_OFFSET flag but the register's value cannot be calculated
using the DWARF_REG_OFFSET method. Create a new flag to indicate that a
different method must be used to calculate the register's value even
though there is no implementation for DWARF_VAL_OFFSET yet; it's mainly
just a place holder.

Signed-off-by: Matt Fleming <matt@console-pimps.org>