Trigger disabling offline renderers on late 2013 Mac Pros

This reads the hw.model string through sysctlbyname and sets the
environment variable QT_MAC_PRO_WEBENGINE_WORKAROUND to tell the
Cocoa platform plugin to not enable offline renderers upon creation
of the platform OpenGL context.

Task-number: QTBUG-70062
Change-Id: I986d9d76a80f96a215f3fcd08b3d47e546682e35
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>