selftests/bpf: Use -O0 instead of -Og in selftests builds

While -Og is designed to work well with debugger, it's still inferior to -O0
in terms of debuggability experience. It will cause some variables to still be
inlined, it will also prevent single-stepping some statements and otherwise
interfere with debugging experience. So switch to -O0 which turns off any
optimization and provides the best debugging experience.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-14-andrii@kernel.org