kcsan, ubsan: Make KCSAN+UBSAN work together

Context:
http://lkml.kernel.org/r/fb7e25d8-aba4-3dcf-7761-cb7ecb3ebb71@infradead.org



Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>