Add bpf_get_stack support

[dthaler]

https://github.com/iovisor/bpf-docs/blob/master/bpf_helpers.rst has:

int bpf_get_stack(struct pt_regs *regs, void *buf, u32 size, u64 flags)

This comes from the BPF_PROG_TYPE_KPROBE context. kprobe support is covered by #206

[dthaler]

Requires kprobe (issue #206) to be done first