selftests/bpf: Use cpu_number only on arches that have it

cpu_number exists only on Intel and aarch64, so skip the test involing
it on other arches. An alternative would be to replace it with an
exported non-ifdefed primitive-typed percpu variable from the common
code, but there appears to be none.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211013160902.428340-2-iii@linux.ibm.com