selftests/bpf: validate .bss section bigger than 8MB is possible now

Add a simple big 16MB array and validate access to the very last byte of
it to make sure that kernel supports > KMALLOC_MAX_SIZE value_size for
BPF array maps (which are backing .bss in this case).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220715053146.1291891-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>