[Backport v3.4-branch] posix: pthread: ensure pthread_key_delete() removes correct key #511

All 3 699 tests pass, 31 skipped in 1h 10m 11s

      7 files   489 suites 1h 10m 11s ⏱️
3 730 tests 3 699 ✔️ 31 💤 0 ❌
3 728 runs 3 699 ✔️ 29 💤 0 ❌

Results for commit 58ba519.