Build v8snapshot with host of matching bitwidth

Use the separate GN toolchain for v8snapshot to match bitwidth of host
and target for that tool.

Task-number: QTBUG-59504
Change-Id: Ic01675a4d4daf4be0327c516195fe87ac16720f2
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>