selftests/bpf: Enable private stack tests for arm64

As arm64 JIT now supports private stack, make sure all relevant tests
run on arm64 architecture.

Relevant tests:

 #415/1   struct_ops_private_stack/private_stack:OK
 #415/2   struct_ops_private_stack/private_stack_fail:OK
 #415/3   struct_ops_private_stack/private_stack_recur:OK
 #415     struct_ops_private_stack:OK
 #549/1   verifier_private_stack/Private stack, single prog:OK
 #549/2   verifier_private_stack/Private stack, subtree > MAX_BPF_STACK:OK
 #549/3   verifier_private_stack/No private stack:OK
 #549/4   verifier_private_stack/Private stack, callback:OK
 #549/5   verifier_private_stack/Private stack, exception in mainprog:OK
 #549/6   verifier_private_stack/Private stack, exception in subprog:OK
 #549/7   verifier_private_stack/Private stack, async callback, not nested:OK
 #549/8   verifier_private_stack/Private stack, async callback, potential nesting:OK
 #549     verifier_private_stack:OK
 Summary: 2/11 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250724120257.7299-4-puranjay@kernel.org