Extend info flag with system/build info
Extend info flag with system/build info

This makes it easier for users to gather system/build info when opening
a bug report.

```
$ bpftrace  --info
System
  OS: Linux 5.3.0-40-generic #32-Ubuntu SMP Fri Jan 31 20:24:34 UTC 2020
  Arch: x86_64

Build
  version: v0.9.4-38-gd079-dirty
  LLVM: 7
  foreach_sym: yes
  unsafe uprobe: no
  btf: no
  bfd: yes

Kernel helpers
  get_current_cgroup_id: yes
  send_signal: yes
  override_return: yes

Kernel features
  Instruction limit: 1000000
  Loop support: yes
```

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace