selftests/bpf: Add negative C tests for kptrs

This uses the newly added SEC("?foo") naming to disable autoload of
programs, and then loads them one by one for the object and verifies
that loading fails and matches the returned error string from verifier.
This is similar to already existing verifier tests but provides coverage
for BPF C.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220511194654.765705-4-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>