libbpf: Remove assumption of single contiguous memory for BTF data

Refactor internals of struct btf to remove assumptions that BTF header, type
data, and string data are layed out contiguously in a memory in a single
memory allocation. Now we have three separate pointers pointing to the start
of each respective are: header, types, strings. In the next patches, these
pointers will be re-assigned to point to independently allocated memory areas,
if BTF needs to be modified.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200926011357.2366158-3-andriin@fb.com