[vax] Convert the backend to MODE_CC so it can be kept in future releases

This is a tracker bug for the convert the vax backend from CC0 to MODE_CC so it can be kept in future releases as there has been interest raised on the NetBSD/VAX mailing list: See: http://mail-index.netbsd.org/port-vax/2020/04/16/msg003460.html For the background of the CC0 transition, see: > https://gcc.gnu.org/wiki/CC0Transition I will be using this bug report to create a bounty on BountySource.com. We have already successfully funded such a conversion for the m68k backend, see #91851. Another campaign is ongoing for the avr backend, see #92729. To test the backend, the emulator SimH can be used to install the VAX port of NetBSD. For that matter, a test image has been provided by John Klos: > http://mail-index.netbsd.org/port-vax/2020/04/18/msg003481.html There is also a generic howto which explains how to set up and install NetBSD/VAX inside SimH: > http://www.netbsd.org/ports/vax/emulator-howto.html SimH is available for immediate installation in openSUSE, Ubuntu, Debian and Fedora. Alternatively, SimH can be obtained from the upstream project: > https://github.com/simh > http://simh.trailing-edge.com/