jenkins: use gcc 8 on the benchmark machines

[richardlau]

Selects gcc/g++ 8 on the benchmark machines when building Node.js 16
or later.

Fixes: #2681

[rvagg]

we're going to need to make sure we have ubuntu-toolchain-r onto 16.04 machines for this to work I think ... or get rid of them entirely

[richardlau]

we're going to need to make sure we have ubuntu-toolchain-r onto 16.04 machines for this to work I think ... or get rid of them entirely

For reference, this happens in

build/ansible/roles/baselayout/tasks/partials/repo/ubuntu1604.yml

Lines 3 to 12 in 3aaa723

	#
	# add PPA for gcc updates
	#
	- name: "repo : add Ubuntu Toolchain PPA"
	apt_repository:
	repo: 'ppa:ubuntu-toolchain-r/test'
	state: present
	update_cache: yes
	register: has_updated_package_repo