libbpf: Treat STV_INTERNAL same as STV_HIDDEN for functions

Do the same global -> static BTF update for global functions with STV_INTERNAL
visibility to turn on static BPF verification mode.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210507054119.270888-7-andrii@kernel.org