tools/bpf: permit selftests/bpf to be built in a different directory

Fix a couple of issues at tools/testing/selftests/bpf/Makefile so
the following command
   make -C tools/testing/selftests/bpf OUTPUT=/home/yhs/tmp
can put the built results into a different directory.

Also add the built binary test_tcpbpf_user in the .gitignore file.

Fixes: 6882804c ("selftests/bpf: add a test for overlapping packet range checks")
Fixes: 9d1f1594 ("bpf: move cgroup_helpers from samples/bpf/ to tools/testing/selftesting/bpf/")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>