libbpf: Recognize __arena global variables.

LLVM automatically places __arena variables into ".arena.1" ELF section.
In order to use such global variables bpf program must include definition
of arena map in ".maps" section, like:
struct {
       __uint(type, BPF_MAP_TYPE_ARENA);
       __uint(map_flags, BPF_F_MMAPABLE);
       __uint(max_entries, 1000);         /* number of pages */
       __ulong(map_extra, 2ull << 44);    /* start of mmap() region */
} arena SEC(".maps");

libbpf recognizes both uses of arena and creates single `struct bpf_map *`
instance in libbpf APIs.
".arena.1" ELF section data is used as initial data image, which is exposed
through skeleton and bpf_map__initial_value() to the user, if they need to tune
it before the load phase. During load phase, this initial image is copied over
into mmap()'ed region corresponding to arena, and discarded.

Few small checks here and there had to be added to make sure this
approach works with bpf_map__initial_value(), mostly due to hard-coded
assump...