ANDROID: binder: remove 32-bit binder interface.

New devices launching with Android P need to use the 64-bit
binder interface, even on 32-bit SoCs [0].

This change removes the Kconfig option to select the 32-bit
binder interface. We don't think this will affect existing
userspace for the following reasons:
1) The latest Android common tree is 4.14, so we don't
   believe any Android devices are on kernels >4.14.
2) Android devices launch on an LTS release and stick with
   it, so we wouldn't expect devices running on <= 4.14 now
   to upgrade to 4.17 or later. But even if they did, they'd
   rebuild the world (kernel + userspace) anyway.
3) Other userspaces like 'anbox' are already using the
   64-bit interface.

Note that this change doesn't remove the 32-bit UAPI
itself; the reason for that is that Android userspace
always uses the latest UAPI headers from upstream, and
userspace retains 32-bit support for devices that are
upgrading. This will be removed as well in 2-3 years,
at which point we can remove the code from the UAPI
as well.

Finally, this change introduces build errors on archs where
64-bit get_user/put_user is not supported, so make binder
unavailable on m68k (which wouldn't want it anyway).

[0]: https://android-review.googlesource.com/c/platform/build/+/595193



Signed-off-by: Martijn Coenen <maco@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>