x86: unification of arch/x86/Makefiles

Unify the 32 and 64 bit specific Makefiles.
The unification was simplest to do in one step although the
readability of the patch suffers a bit from this.

Noteworthy remarks on the unification:
- The 64 bit cpu stuff should be moved to Makefile_32.cpu
  but I did not feel confident doing it due to subtle differences
- The use of cflags-y were abandoned since we have seen one bug where
  we did wrong due to missing assignment to KBUILD_CFLAGS.
  The cc-option marcro uses KBUILD_CFLAGS.
- The "No need to remake" line are deleted. It caused "make -B" to fail
- For 64 bit the sub architecture stuff is not used.
- The way head64.o is specified could be nicer - but it awaits the
  introduction of head32.o (which seems like a win to introduce for readability)
- Patch is checkpatch clean

Patch is tested by doing a defconfig build for i386 and x86_64 and in both
cases monitoring that only relevant files were recompiled when applying
the patch.

[ mingo@elte.hu: build fix ]

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>