selftests/bpf: Revert CO-RE removal in test_ksyms_weak.

The commit 087cba799ced ("selftests/bpf: Add weak/typeless ksym test for light skeleton")
added test_ksyms_weak to light skeleton testing, but remove CO-RE access.
Revert that part of commit, since light skeleton can use CO-RE in the kernel.

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

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

···
 	/* tests existing symbols. */
 	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 0);
 	if (rq)
-		out__existing_typed = 0;
+		out__existing_typed = rq->cpu;
 	out__existing_typeless = (__u64)&bpf_prog_active;
 
 	/* tests non-existent symbols. */