net-test: allow bpf on 32-bit userspace with 64-bit kernel

This resolves:
  Running /host/all_tests.py

  Traceback (most recent call last):
    File "/host/all_tests.py", line 51, in <module>
      import_module(name)
    File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
      __import__(name)
    File "/host/bpf_test.py", line 26, in <module>
      from bpf import *  # pylint: disable=wildcard-import
    File "/host/bpf.py", line 41, in <module>
      }[os.uname()[4] + "-" + platform.architecture()[0]]
  KeyError: 'x86_64-32bit'

Bug: 126465735
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I64f246a4fd34c8a84e9c349957060d7d1be18175