Put common override code into gv_override

Put common override code into gv_override

When I moved the three occurrences of this code in op.c into a static
function, I did not realise at the time that it also occurred thre
etimes in toke.c.

So now it is in a new non-static function in gv.c.

Only two of the instances in toke.c could be changed to use this func-
tion, as the otherwise is a little different.  I couldn’t see a simple
way of factoring its requirements in.