selftests/bpf: Add tests for ld_{abs,ind} failure path in subprogs

Extend the verifier_ld_ind BPF selftests with subprogs containing
ld_{abs,ind} and craft the test in a way where the invalid register
read is rejected in the fixed case. Also add a success case each,
and add additional coverage related to the BTF return type enforcement.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_ld_ind
  [...]
  #611/1   verifier_ld_ind/ld_ind: check calling conv, r1:OK
  #611/2   verifier_ld_ind/ld_ind: check calling conv, r1 @unpriv:OK
  #611/3   verifier_ld_ind/ld_ind: check calling conv, r2:OK
  #611/4   verifier_ld_ind/ld_ind: check calling conv, r2 @unpriv:OK
  #611/5   verifier_ld_ind/ld_ind: check calling conv, r3:OK
  #611/6   verifier_ld_ind/ld_ind: check calling conv, r3 @unpriv:OK
  #611/7   verifier_ld_ind/ld_ind: check calling conv, r4:OK
  #611/8   verifier_ld_ind/ld_ind: check calling conv, r4 @unpriv:OK
  #611/9   verifier_ld_ind/ld_ind: check calling conv, r5:OK
  #611/10  verifier_ld_ind/ld_ind: check calling conv, r5 @unpriv:OK
  #611/11  verifier_ld_ind/ld_ind: check calling conv, r7:OK
  #611/12  verifier_ld_ind/ld_ind: check calling conv, r7 @unpriv:OK
  #611/13  verifier_ld_ind/ld_abs: subprog early exit on ld_abs failure:OK
  #611/14  verifier_ld_ind/ld_ind: subprog early exit on ld_ind failure:OK
  #611/15  verifier_ld_ind/ld_abs: subprog with both paths safe:OK
  #611/16  verifier_ld_ind/ld_ind: subprog with both paths safe:OK
  #611/17  verifier_ld_ind/ld_abs: reject void return subprog:OK
  #611/18  verifier_ld_ind/ld_ind: reject void return subprog:OK
  #611     verifier_ld_ind:OK
  Summary: 1/18 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260408191242.526279-4-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>