x86: re-add stack alignment check

Commit 279840d5ea ("x86/boot: install trap handlers much earlier on
boot"), perhaps not really intentionally, removed this check. Add it
back,
- preventing it to trigger before any output is set up,
- accompanying it with a (weaker, due to its open coding of what
  get_stack_bottom() does) build time check.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
While not strictly needed for 4.8, I thought I'd still submit it as a
possible candidate.

--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -643,6 +643,11 @@ void load_system_tables(void)
 		.limit = (IDT_ENTRIES * sizeof(idt_entry_t)) - 1,
 	};
 
+	/* Bottom-of-stack must be 16-byte aligned! */
+	BUILD_BUG_ON((sizeof(struct cpu_info) -
+		      offsetof(struct cpu_info, guest_cpu_user_regs.es)) & 0xf);
+	BUG_ON(system_state != SYS_STATE_early_boot && (stack_bottom & 0xf));
+
 	/* Main stack for interrupts/exceptions. */
 	tss->rsp0 = stack_bottom;
 	tss->bitmap = IOBMP_INVALID_OFFSET;