samples: bpf: Refactor test_overhead program with libbpf

This commit refactors the existing program with libbpf bpf loader.
Since the kprobe, tracepoint and raw_tracepoint bpf program can be
attached with single bpf_program__attach() interface, so the
corresponding function of libbpf is used here.

Rather than specifying the number of cpus inside the code, this commit
uses the number of available cpus with _SC_NPROCESSORS_ONLN.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201124090310.24374-6-danieltimlee@gmail.com