selftests/bpf: Introduce experimental bpf_in_interrupt()

Filtering pid_tgid is meanlingless when the current task is preempted by
an interrupt.

To address this, introduce 'bpf_in_interrupt()' helper function, which
allows BPF programs to determine whether they are executing in interrupt
context.

'get_preempt_count()':

* On x86, '*(int *) bpf_this_cpu_ptr(&__preempt_count)'.
* On arm64, 'bpf_get_current_task_btf()->thread_info.preempt.count'.

Then 'bpf_in_interrupt()' will be:

* If !PREEMPT_RT, 'get_preempt_count() & (NMI_MASK | HARDIRQ_MASK
  | SOFTIRQ_MASK)'.
* If PREEMPT_RT, '(get_preempt_count() & (NMI_MASK | HARDIRQ_MASK))
  | (bpf_get_current_task_btf()->softirq_disable_cnt & SOFTIRQ_MASK)'.

As for other archs, it can be added support by updating
'get_preempt_count()'.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20250903140438.59517-2-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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