libbpf: Refactor internals of BTF type index

Refactor implementation of internal BTF type index to not use direct pointers.
Instead it uses offset relative to the start of types data section. This
allows for types data to be reallocatable, enabling implementation of
modifiable BTF.

As now getting type by ID has an extra indirection step, convert all internal
type lookups to a new helper btf_type_id(), that returns non-const pointer to
a type by its ID.

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/20200926011357.2366158-2-andriin@fb.com