merge gcc 7.4 into gcc.old part 3 (last?)

delete GCC 6.5. every platform has been on GCC 7 for a while now.

merge GCC 6.5 into gcc.old (properly this time.)

import GCC 6.4 srcs into gcc.old in preparation for GCC 7.x.

these are netbsd GCC srcs from 20180801.