Currently, vmlinux.lds.s is generated by the top-level Makefile.
Unfortunately, this causes the automatic CONFIG dependencies to
not work correctly, the reason being that make caches the timestamps
of include/config/* so even after split-include updated the
timestamps, make still uses the cached stamps to decide whether to
rebuild vmlinux.lds.s.

The simple fix is to move generation of vmlinux.lds.s into the
arch/$(ARCH)/kernel subdirectories, where we build other files
for the final link (like head.o) anyway. This also means some
special code in the top-level Makefile for preprocessing can go
away, since we now just use the standard rules during the recursive
phase.

This patches fixes up all archs for this change (untested).