When a signal is raised in a leaf function (that hasn't backed up
the Lr register itself), we need to restore both Pc and Lr separately.
This doesn't seem to be possible via libunwind, so add a special case
unwinding rule for this function.

As we have a special case for the function, we could also just as well
skip normal dwarf unwinding for this function altogether and just
restore the full context from the stack.
Signed-off-by: Martin Storsjo <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>