One more point: suggestions along the lines of "patch the kernel" are no good in this case. Ever heard of git bisect? Once the proposed Makefile changes are far enough in the past (5 years or so) - fine, but until then you've just made life considerably more unpleasant for those of us who do kernel development on debian boxen. Sure, it can be worked around, by remembering to pass the damn thing in kbuild flags. Delightful, that...
The bug is in gcc-6. Reassigning it elsewhere doesn't change the fact that it breaks one of the normal uses of gcc and no amount of changes in debian packaging of kernel source is going to affect that.