VxWorks 7 support in Qt 5.15.10

Existing VxWorks support was done for Qt 5.12. Now this port is being enhanced to support Qt 5.15.10. This epic covers the related tickets need for VxWorks 7 support in Qt 5.15. Qt6 support is a separate activity not covered in this ticket.

Current Qt 5.15 code base has diverged from the Qt 5.12 port, hence a rebase is first needed from 5.12to include all 5.12 changes in 5.15. This wodl also mean that previous SRs are invlcuded, in particular SR540 support is required also for the 5.15.10 port fone in this ticket in addition to SR660 where the latest changes are provided.

As a separate activity, to decrease the work load with VxWorks future porting activities VxWorks integration should be merged into QtBase.