The aarch64 startup benchmark is reporting very unstable results for Qt 5.12

On August 1st and then again on August 13th, 2019, the startup times as measured by the aarch64 QML start-up benchmark have significantly increased. At the same time the variation between different runs of the same benchmark has increased to a point where the data seems almost meaningless. Either the benchmark code is broken or we have changed something that lets the start-up time vary wildly between 200ms and 1min.