For-loops do not handle BPF context

[ajor]

Latest bpftrace will SEGV when compiling scripts which attempt to access the BPF context from the body of a for-loop:

kprobe:do_nanosleep
{
  @map[0] = 1;
  for ($kv : @map) {
    arg0; // requires ctx access
  }
}

I think BPF context will have to be passed into the callback subprogram, similarly to variables (#3014).