sh: Allow multiple stack unwinders to be setup

Provide an interface for registering stack unwinders, where each
unwinder is given a rating that describes its accuracy and
complexity. The more accurate an unwinder is, the more complex it is.

If a the current stack unwinder faults, then the stack unwinder with the
next highest accuracy will be used in its place (provided one is
available). For example, this allows unwinders, such as the DWARF
unwinder, to liberally sprinkle BUG()s to catch badly formed DWARF debug
info.

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