index_v1: restore nativecode field

Alexander Martinzrequested to merge
amartinz/fdroidserver:v1-add-nativecode into master

This got removed with commit f0eedc3a which results in issues with clients using index v1.

These clients may end up offering the wrong arch for the device.

closes #1327 (closed)

Edited by Hans-Christoph Steiner

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