selftests/bpf: Fix test_ksyms on non-SMP kernels

On non-SMP kernels __per_cpu_start is not 0, so look it up in kallsyms.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200910171336.3161995-1-iii@linux.ibm.com