selftests/bpf: Fix arena_spin_lock selftest failure

selftests/bpf: Fix arena_spin_lock selftest failure

For systems having CONFIG_NR_CPUS set to > 1024 in kernel config
the selftest fails as arena_spin_lock_irqsave() returns EOPNOTSUPP.
(eg - incase of powerpc default value for CONFIG_NR_CPUS is 8192)

The selftest is skipped incase bpf program returns EOPNOTSUPP,
with a descriptive message logged.

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Link: https://lore.kernel.org/r/20250913091337.1841916-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>