arch/x86: Drop Kconfig AP_SIPI_VECTOR

This was used to check romcc-built bootblock and romstage
agree about the location of 16-bit entrypoint. There was
no need to customize it as bootblock size requirement did
not grow. Just check for a fixed location at 4 GiB - 4 KiB.

With C_ENVIRONMENT_BOOTBLOCK we can have a proper symbol
for the purpose, since it appears in the same compilation
unit. It will adjust if C_ENV_BOOTBLOCK_SIZE changes.

Change-Id: I93f3c37e78ba587455c804de8c57e7e06832a81f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/30854
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

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

@@ -28,6 +28,13 @@
  */
 
 #include <arch/rom_segs.h>
+
+#if IS_ENABLED(CONFIG_SIPI_VECTOR_IN_ROM)
+/* Symbol _start16bit must be aligned to 4kB to start AP CPUs with
+ * Startup IPI message without RAM.
+ */
+.align 4096
+#endif
 .code16
 .globl _start16bit
 .type _start16bit, @function

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

@@ -1,2 +1,3 @@
 	gdtptr16_offset = gdtptr16 & 0xffff;
 	nullidt_offset = nullidt & 0xffff;
+	ap_sipi_vector_in_rom = (_start16bit >> 12) & 0xff;