We need a macro detection for VxWorks's stdlib (if it's not libc++ or libstdc++)

Code that should be checking for a particular C++ stdlib is atm checking for Q_OS_VXWORKS. This is wrong. It needs to check for a particular standard library implementation instead.

Reference: https://codereview.qt-project.org/c/qt/qtbase/+/610478/comment/c953c477_10b79ee0/

Acceptance Criteria:

• find out which vendor VxWorks's stdlib comes from
• find out which C macro reliably detects said vendor's stdlib and patch qcompare.h to use it when selecting the enum value of unordered