Drop the manually defined *_XDP constants which were added to glibc.
Install python3 instead of python because glibc's confugure script now
requires at least 3.4.

Change-Id: Ic19ff4d8aa4dbadeb5438e331ccafeafc2cc9925
Reviewed-on: https://go-review.googlesource.com/c/160957
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>