ndk-build can't differentiate between .o files when only the extension has changed

For the given Android.mk:

```makefile
LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE := foo
LOCAL_SRC_FILES := foo.c foo.cpp
include $(BUILD_EXECUTABLE)
```

ndk-build replaces the extension with .o rather than appending it, so foo.o will be either the object file for foo.c or foo.cpp depending on who wins the race.

Should be a trivial fix (use foo.c.o and foo.cpp.o instead of foo.o), but it's also low priority. Would be a good bug for an external contributor looking to get started.

Start with the Android.mk example and trace the ndk-build object-file naming rules used for LOCAL_SRC_FILES. Verify the change with both foo.c and foo.cpp, and consider how the build confirms that distinct object files are produced instead of both mapping to foo.o.

Written by the indexing model from the issue text.