selftests/bpf: Add powerpc support for get_preempt_count() in selftest

selftests/bpf: Add powerpc support for get_preempt_count() in selftest

get_preempt_count() is enabled to return preempt_count for powerpc,
so that bpf_in_interrupt()/bpf_in_nmi()/bpf_in_serving_softirq()/
bpf_in_task()/bpf_in_hardirq()/get_preempt_count() works for
powerpc as well.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Link: https://lore.kernel.org/r/20260212092558.370623-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>