selftests/bpf: Enable private stack tests for powerpc64

selftests/bpf: Enable private stack tests for powerpc64

With support of private stack, relevant tests must pass
on powerpc64.

#./test_progs -t struct_ops_private_stack
#434/1   struct_ops_private_stack/private_stack:OK
#434/2   struct_ops_private_stack/private_stack_fail:OK
#434/3   struct_ops_private_stack/private_stack_recur:OK
#434     struct_ops_private_stack:OK
Summary: 1/3 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260401103215.104438-2-adubey@linux.ibm.com