selftests/bpf: move get_ksyms and get_addrs to trace_helpers.c

selftests/bpf: move get_ksyms and get_addrs to trace_helpers.c

We need to get all the kernel function that can be traced sometimes, so we
move the get_syms() and get_addrs() in kprobe_multi_test.c to
trace_helpers.c and rename it to bpf_get_ksyms() and bpf_get_addrs().

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Link: https://lore.kernel.org/r/20250904021011.14069-2-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>