Merge branch 'cleanup-selftests-bpf-makefile'

Andrii Nakryiko says:

====================
This patch set extensively revamps selftests/bpf's Makefile to generalize test
runner concept and apply it uniformly to test_maps and test_progs test
runners, along with test_progs' few build "flavors", exercising various ways
to build BPF programs.

As we do that, we fix dependencies between various phases of test runners, and
simplify some one-off rules and dependencies currently present in Makefile.
test_progs' flavors are now built into root $(OUTPUT) directory and can be run
without any extra steps right from there. E.g., test_progs-alu32 is built and
is supposed to be run from $(OUTPUT). It will cd into alu32/ subdirectory to
load correct set of BPF object files (which are different from the ones built
for test_progs).

Outline:
- patch #1 teaches test_progs about flavor sub-directories;
- patch #2 fixes one of CO-RE tests to not depend strictly on process name;
- patch #3 changes test_maps's usage of map_tests/tests.h to be the same as
  test_progs' one;
- patch #4 adds convenient short `make test_progs`-like targets to build only
  individual tests, if necessary;
- patch #5 is a main patch in the series; it uses a bunch of make magic
  (mainly $(call) and $(eval)) to define test runner "skeleton" and apply it
  to 4 different test runners, lots more details in corresponding commit
  description;
- patch #6 does a bit of post-clean up for test_queue_map and test_stack_map
  BPF programs;
- patch #7 cleans up test_libbpf.sh/test_libbpf_open superseded by test_progs.

v3->v4:
- remove accidentally checked in binaries;

v2->v3:
- drop test_xdp.o mixed compilation mode, remove test_libbpf.sh (Alexei);

v1->v2:
- drop test_progs-native causing compilation failures due to
  __builtin_preserve_field_access, add back test_xdp.o override, which will
  now emit rule re-definition warning.
====================

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