cpu/intel/car/p4-netburst: Add assert for SIPI_VECTOR_IN_ROM

Location of _start16bit in entry16.inc is about to see some changes,
lets make sure they don't break the alignment requirement here.

Change-Id: Id8a0964982387e5321e8c89254922e1242cf85ee
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37894
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/cpu/x86/16bit/entry16.ld

@@ -1,3 +1,23 @@
-	gdtptr16_offset = gdtptr16 & 0xffff;
-	nullidt_offset = nullidt & 0xffff;
-	ap_sipi_vector_in_rom = (_start16bit >> 12) & 0xff;
+/*
+ * This file is part of the coreboot project.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+gdtptr16_offset = gdtptr16 & 0xffff;
+nullidt_offset = nullidt & 0xffff;
+
+/* Symbol _start16bit must be aligned to 4kB to start AP CPUs with
+ * Startup IPI message without RAM.
+ */
+#if CONFIG(SIPI_VECTOR_IN_ROM)
+_bogus = ASSERT((_start16bit & 0xfff) == 0, "Symbol _start16bit is not at 4 KiB boundary");
+ap_sipi_vector_in_rom = (_start16bit >> 12) & 0xff;
+#endif