selftests/bpf: Define SYS_NANOSLEEP_KPROBE_NAME for aarch64

attach_probe selftest fails on aarch64 with `failed to create kprobe
'sys_nanosleep+0x0' perf event: No such file or directory`. This is
because, like on several other architectures, nanosleep has a prefix.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20220404142101.27900-1-iii@linux.ibm.com