I doubt it affects ABI, but let's not take the risk.

Instead, just compile the uunxapi.cxx file (which has no public API)
as Objective-C++, as that is now the only one where Objective-C API is
used.

This reverts part of d22c94db

Change-Id: I240dbb36529c419b81b062f6fe860a9e621c2400