Add __NR_bpf constant for i686.

This adds support for running the net tests on 32-bit x86.

It was tested using the 64-bit cuttlefish kernel (4.9).

Bug: 79532682
Test: vts_kernel_tests ran to completion on cuttlefish
Change-Id: I5a8ade3a2292734001dd6e85f9c6b5cefc8de9c1
Signed-off-by: Alistair Strachan <astrachan@google.com>