Give more resources to build huge ports in Poudriere

Summary:
Ports like compilers require a lot of time to build and block the queue.

As such, this change allows them to use several cores to speed up build,
as the other core are probably idling waiting for the huge port.

Ref T2039.

Test Plan: Tested on WindRiver

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2039

Differential Revision: https://devcentral.nasqueron.org/D3489