arm: Prevent compilation of old, experimental SMP support

The ARM SMP feature was added a long time ago and has never really been
used by anyone since. We are still always compiling cpu_info() even
though we don't use it, and it makes some dangerous assumptions about
stack alignment that are not guaranteed anywhere.

I'm planning to change the way the stack boundaries are defined. Rather
than trying to work that into this unsafe, unused and hard to test
feature, I think we should just seal it off with police tape and make
sure that if anyone ever tries to use it again (which currently seems
unlikely), they get forced to do their due diligence on making sure it
works as intended.

BUG=None
TEST=Compiled Veyron_Pinky.

Change-Id: I8a60bd30e8b27a22bb3da68ca84daea99424dee9
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/219680
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>

src/arch/arm/Makefile.inc

@@ -117,7 +117,7 @@ ifeq ($(CONFIG_ARCH_RAMSTAGE_ARM),y)
 
 ramstage-y += stages.c
 ramstage-y += div0.c
-ramstage-y += cpu.c
+ramstage-$(CONFIG_COOP_MULTITASKING) += cpu.c
 ramstage-y += eabi_compat.c
 ramstage-y += boot.c
 ramstage-y += tables.c

src/arch/arm/cpu.c

@@ -34,6 +34,11 @@
  */
 struct cpu_info *cpu_info(void)
 {
+#error "This is BROKEN! ARM stacks are currently not guaranteed to be " \
+       "STACK_SIZE-aligned in any way. If you ever plan to revive this " \
+       "feature, make sure you add the proper assertions " \
+       "(and maybe consider revising the whole thing to work closer to what " \
+       "arm64 is doing now)."
 	uintptr_t addr = ALIGN((uintptr_t)__builtin_frame_address(0),
 		CONFIG_STACK_SIZE);
 	addr -= sizeof(struct cpu_info);