selftests/bpf: make BPF compiler flags stricter

We recently added -Wuninitialized, but it's not enough to catch various
silly mistakes or omissions. Let's go all the way to -Wall, just like we
do for user-space code.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230309054015.4068562-5-andrii@kernel.org


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