binder: Allow 64-bit kernel to run binder with backward compatibility

Some targets may run with a 64-bit kernel, but 32-bit userspace
with binder v7.  Allow 64-bit kernel configs for such targets to
enable ANDROID_BINDER_IPC_32BIT for backward compatibility.

Change-Id: I24bf3af1bdcb8e3363911e1f4ff7b3dc05ee37c5
Signed-off-by: Ajay Dudani <adudani@codeaurora.org>
Signed-off-by: David Ng <dave@codeaurora.org>