"BPF_STX uses reserved fields" validation error when using __sync_lock_test_and_set

Hi, in an eBPF program that i'm loading with gobpf I need to perform atomic increments and atomic set. For the first, I use `__sync_fetch_and_add` with no issues, however if I try to use `__sync_lock_test_and_set` for the latter, while compilation passes, I get a runtime error from the validator:

```
BPF_STX uses reserved fields
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
```

How do I perform atomic set of a variable so that i don't have to use PER_CPU arrays?

eBPF program compiled with clang 12.0.1, running on 5.8.0-53-generic ( x86_64 )

Start with the eBPF program using __sync_lock_test_and_set and inspect the generated BPF instructions around the failing BPF_STX validation. Compare this with the working __sync_fetch_and_add case on clang 12.0.1 and Linux 5.8. Done means identifying a supported atomic-set approach that validates without using PER_CPU arrays.

Written by the indexing model from the issue text.