Add MKFILE_DEPS to compiler-rt target

Currently when you run `make -jN` it's likely that you'll remove compiler-rt and
then it won't get cp'd back into the right place. I believe the reason for this
is that the compiler-rt library target never got updated so make decided it
never needed to copy the files back into place. The files were all there at the
beginning of `make`, but then we may clean out the stage0 versions if we unzip
the snapshot again.