Add cross-compilation support to eBPF samples

These patches fix issues seen when cross-compiling eBPF samples on arm64. Compared to [1], I dropped the controversial inline-asm patch pending further discussion on the right way to do it. However these patches are still a step in the right direction and I wanted them to get in before the more controversial bit.

[1] https://lkml.org/lkml/2017/8/7/417

Joel Fernandes (4):
samples/bpf: Use getppid instead of getpgrp for array map stress samples/bpf: Enable cross compiler support samples/bpf: Fix pt_regs issues when cross-compiling samples/bpf: Add documentation on cross compilation

 samples/bpf/Makefile                      |  7 +++-
 samples/bpf/README.rst                    | 10 ++++++
 samples/bpf/map_perf_test_kern.c          |  2 +-
 samples/bpf/map_perf_test_user.c          |  2 +-
 tools/testing/selftests/bpf/bpf_helpers.h | 56 +++++++++++++++++++++++++++----
 5 files changed, 67 insertions(+), 10 deletions(-)

2.14.1.581.gf28d330327-goog

All patches in this set look good to me.
When you repost without changes after net-next reopens pls keep my:

Hi Alexei,

Thanks a lot! will do.

Regards,
Joel