arch/x86: Add a common romstage entry

It might be possible to have this used for more than x86, but that
will be for a later commit.

Change-Id: I4968364a95b5c69c21d3915d302d23e6f1ca182f
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55067
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/soc/example/min86/romstage.c

@@ -1,7 +1,10 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-#include <arch/cpu.h>
+#include <romstage_common.h>
+#include <halt.h>
 
-asmlinkage void car_stage_entry(void)
+void __noreturn romstage_main(void)
 {
+	/* Needed for __noreturn */
+	halt();
 }