Merge branch 'bpf-prog-build'

Jiong Wang says:

====================
This set improves bpf object file related rules in selftests Makefile.
  - tell git to ignore the build dir "alu32".
  - extend sub-register mode compilation to all bpf object files to give
    LLVM compiler bpf back-end more exercise.
  - auto-generate bpf kernel object file list.
  - relax sub-register mode compilation criteria.

v1 -> v2:
  - rename "kern_progs" to "progs". (Alexei)
  - spin a new patch to remove build server kernel requirement for
    sub-register mode compilation (Alexei)
  - rebase on top of KaFai’s latest "test_sock_fields" patch set.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>