libbpf: Allow decimal offset for kprobes

Allow to specify decimal offset in SEC macro, like:
  SEC("kprobe/bpf_fentry_test7+5")

Add selftest for that.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20210721215810.889975-3-jolsa@kernel.org