Flamegraph with no samples been generated

Team

### Environment

* Arthas version:
* Operating System version: Ubuntu 16.04 4.15.0-107-generic
* Java version of target JVM: 1.8.0_212
* Java version of JVM used to attach: 1.8.0_212

### Steps to reproduce this issue

1. To create a flamegraph used the below command
2. profiler --alluser -d 300 -e cpu -f rtcs_cpu.svg start
OK
profiler will silent stop after 300 seconds.
profiler output file will be: rtcs_cpu.svg
3. file has been generated but no samples file is empty.

### Expected Result

Flamegraph

### Actual Result

Flamegraph with no data

Do we need linux perf to generate flamegrpah for --all-user option
Is perf mandatory for --all-user option

Start by reproducing the issue with `profiler --alluser -d 300 -e cpu -f rtcs_cpu.svg` on the stated Ubuntu and Java versions. Check whether the generated flamegraph contains samples and determine whether Linux perf is required for the `--alluser` option; done means the command produces a non-empty flamegraph or documents the requirement clearly.

Written by the indexing model from the issue text.