The entire libs/ directory is built and constructed automatically with
the build_libs.sh script.  Don't attempt to modify manually.  To rebuild
the native code, make the following change to the NDK to pass through
the target architecture, and then run build_libs.sh:

build/core/build-binary.mk:LOCAL_CFLAGS := -DANDROID -D__ANDROID_ARCH__=\"$(TARGET_ARCH_ABI)\" $(LOCAL_CFLAGS)