selftests/bpf: Add a selftest with not-8-byte aligned BPF_ST

selftests/bpf: Add a selftest with not-8-byte aligned BPF_ST

Add a selftest with a 4 bytes BPF_ST of 0 where the store is not
8-byte aligned. The goal is to ensure that STACK_ZERO is properly
marked in stack slots and the STACK_ZERO value can propagate
properly during the load.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240110051355.2737232-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>