Parsing of non-fixed (&optional, &key and &rest) arguments used to
fail when there were more fixed arguments than slots in the stack
frame. Fix this on x86oids by copying non-fixed arguments in the
correct direction, depending on whether there are more fixed args
or stack frame slots.
This bug is more visible on x86oids since 3b98d3 (Smaller stack
frames on x86oids), but may still plague other platforms. These
platforms still have larger initial stack frame size (8 slots), so
the issue remains as hard to trigger as it's been for more than a
Reported by Jan Moringen, and reduced by Stas Boukarev.