Add pahole utility needed for BTF generation in our kernels

[sdimitro]

Motivation

bpftrace has had BTF support for a while (ref: https://github.com/iovisor/bpftrace/blob/master/docs/reference_guide.md#btf-support). We currently have in-place a few hacks to include ZFS (and other?) headers in our bpftrace package so we can use the type information on our scripts. If we had BTF info on our kernels we would no longer need those hacks for ZFS (and any other kernel modules really).

This Patch

This PR includes the pahole(1) utilty that is part of the dwarves package from Canonical. This utility will be automatically used by our kernel builds when we enable the CONFIG_DEBUG_INFO_BTF config flag on our kernels, to generate BTF info.

Testing

TBA

[sebroy]

I'm starting to get excited about the prospect of leveraging BTF!

[sdimitro]

The pahole utility that comes with 18.04 is too old for the kernel build which fails with the following error when trying to generate BTF:

BTF: .tmp_vmlinux.btf: pahole version v1.9 is too old, need at least v1.13
Failed to generate BTF for vmlinux
Try to disable CONFIG_DEBUG_INFO_BTF

We'll revisit this after we move to 20.04, which I think should be soon.