Do not enable DEBUG_FILES in stages 1. through 3. of the build

Blindly building debugging symbols into the toolchain slows down builds
considerably and eats up a lot more resources than required, both in
terms of memory and disk I/O

This change has no noticeable effect for those who do not build with
MK_DEBUG_FILES=no (which is the default)

Sponsored by: EMC / Isilon Storage Division
Remove leftover MK_VI == no from previous work that's no longer needed