selftests/bpf: validate eliminated global subprog is not freplaceable

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 2afae08c9dcb ("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 <[email protected]>
Reviewed-by: Alan Maguire <[email protected]>
Suggested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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