selftests: bpf: fix compilation with out-of-tree $(OUTPUT)

A bunch of related changes lumped together:
* Create prog_tests and verifier output directories; these don't exist with
  out-of-tree $(OUTPUT)
* Add missing -I (via separate TEST_{PROGS,VERIFIER}_CFLAGS) for the main tree
  ($(PWD) != $(OUTPUT) for out-of-tree)
* Add libbpf.a dependency for test_progs_32 (parallel make fails otherwise)
* Add missing "; \" after "cd" when generating test.h headers

Tested by:
$ alias m="make -s -j$(nproc)"
$ m -C tools/testing/selftests/bpf/ clean
$ m -C tools/lib/bpf/ clean
$ rm -rf xxx; mkdir xxx; m -C tools/testing/selftests/bpf/ OUTPUT=$PWD/xxx
$ m -C tools/testing/selftests/bpf/

Fixes: 3f306588 ("selftests: bpf: break up test_progs - preparations")
Fixes: 2dfb4012 ("selftests: bpf: prepare for break up of verifier tests")
Fixes: 3ef84346 ("selftests: bpf: makefile support sub-register code-gen test mode")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>