Provide targets to build rvct4 for symbian-sbsv2 mkspec generated makefiles.

Qmake should generate makefile targets for building with RVCT4. One possible solution to do this is to handle RVCT similarly to GCCE, i.e. attempt to determine desired RVCT version and generate proper sbs commands for release-armv6 etc. targets accordingly. However, if possible, a better option would be to check if the used Symbian SDK is RVCT enabled and generate RVCT4 commands if that is the case.