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 <asavkov@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220816055231.717006-1-asavkov@redhat.com