arch/x86: Link gdt_init.S into bootblock

Followup work forces gdtptr and gdt towards the top of
bootblock. They need to be realmode-addressable, i.e.
within top 64 KiB or same segment with .reset.

Change-Id: Ib6f23b2808d0a7e0d277d00a9b0f30c49fdefdd5
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47965
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

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

@@ -124,7 +124,7 @@ _start16bit:
 	ljmpl	$ROM_CODE_SEG, $__protected_start
 
 	/**
-	 * The gdt is defined in entry32.inc, it has a 4 Gb code segment
+	 * The gdt is defined in gdt_init.S, it has a 4 Gb code segment
 	 * at 0x08, and a 4 GB data segment at 0x10;
 	 */
 __gdtptr:

src/cpu/x86/32bit/entry32.inc

@@ -4,7 +4,6 @@
 
 #include <arch/rom_segs.h>
 #include <cpu/x86/post_code.h>
-#include <arch/x86/gdt_init.S>
 
 	.code32
 /*