selftests/bpf: Fix attach point for non-x86 arches in test_progs/lsm

Use SYS_PREFIX macro from bpf_misc.h instead of hard-coded '__x64_'
prefix for sys_setdomainname attach point in lsm test.

Signed-off-by: Artem Savkov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/DENYLIST.s390x

···
 test_bprm_opts                           # failed to auto-attach program 'secure_exec': -524                           (trampoline)
 test_ima                                 # failed to auto-attach program 'ima': -524                                   (trampoline)
 test_local_storage                       # failed to auto-attach program 'unlink_hook': -524                           (trampoline)
-test_lsm                                 # failed to find kernel BTF type ID of '__x64_sys_setdomainname': -3          (?)
+test_lsm                                 # attach unexpected error: -524                                               (trampoline)
 test_overhead                            # attach_fentry unexpected error: -524                                        (trampoline)
 test_profiler                            # unknown func bpf_probe_read_str#45                                          (overlapping)
 timer                                    # failed to auto-attach program 'test1': -524                                 (trampoline)

tools/testing/selftests/bpf/progs/lsm.c

···
  * Copyright 2020 Google LLC.
  */
 
+#include "bpf_misc.h"
 #include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
···
 
 int copy_test = 0;
 
-SEC("fentry.s/__x64_sys_setdomainname")
+SEC("fentry.s/" SYS_PREFIX "sys_setdomainname")
 int BPF_PROG(test_sys_setdomainname, struct pt_regs *regs)
 {
 	void *ptr = (void *)PT_REGS_PARM1(regs);