selftests/bpf: Undo GCC-specific bpf_helpers.h changes

Having GCC provide its own bpf-helper.h is not the right approach and is
going to be changed. Undo bpf_helpers.h change before moving
bpf_helpers.h into libbpf.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20191008175942.1769476-2-andriin@fb.com