Enabling it will force usage of '-O3' flag when building Qt.
If the platform has no '-O3' flag, use '-O2' as a fallback.
Task-number: QTBUG-86866
Change-Id: If13f7de954ba5c01dc9634f06a85529828fe90a9
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>