selftests/bpf: Switch to ".bss"/".rodata"/".data" lookups for internal maps

Utilize libbpf's feature of allowing to lookup internal maps by their
ELF section names. No need to guess or calculate the exact truncated
prefix taken from the object name.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211021014404.2635234-11-andrii@kernel.org