Add selftest that establishes dead code-eliminated valid global subprog
(global_dead) and makes sure that it's not possible to freplace it, as
it's effectively not there. This test will fail with unexpected success
before 2afae08c ("bpf: Validate global subprogs lazily").

v2->v3:
  - add missing err assignment (Alan);
  - undo unnecessary signature changes in verifier_global_subprogs.c (Eduard);
v1->v2:
  - don't rely on assembly output in verifier log, which changes between
    compiler versions (CI).
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20231211174131.2324306-1-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>