selftest/bpf: Add a test case for ld_imm64 copy logic.

Add a test case to exercise {btf_id, btf_obj_fd} copy logic between ld_imm64 insns.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230319203014.55866-2-alexei.starovoitov@gmail.com

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

@@ -37,7 +37,7 @@ int pass_handler(const void *ctx)
 
 	/* tests existing symbols. */
 	rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 0);
-	if (rq)
+	if (rq && bpf_ksym_exists(&runqueues))
 		out__existing_typed = rq->cpu;
 	out__existing_typeless = (__u64)&bpf_prog_active;