bpffeature: fix helper detection for older kernels
bpffeature: fix helper detection for older kernels

on stretch:

```
(gdb) p logbuf
$2 = "\000: (85) call 4\nR1 type=ctx expected=fp\n", '\000' <repeats 4056 times>
(gdb) p/x logbuf
$3 = {0x0, 0x3a, 0x20, 0x28
```

Lets hope checking 8 bytes is enough...

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