[Bug libobjc/78697] --with-target-bdw-gc-include shouldn't be required to be per multilib

[doko at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78697

--- Comment #2 from Matthias Klose <doko at gcc dot gnu.org> ---
patch posted at
https://gcc.gnu.org/ml/gcc-patches/2016-12/msg02003.html