libbpf: Sanitize global functions

In case the kernel doesn't support BTF_FUNC_GLOBAL sanitize BTF produced by the
compiler for global functions.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200110064124.1760511-2-ast@kernel.org