arch/x86: Use defines for GDT segments

Stop using magic values and use defines for Global Descriptor Table
(GDT) offsets. Use the existing defines from the corresponding headers.

Change-Id: I40c15f6341bdef9cd457619ec81e7ac624ec2d63
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87254
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/device/oprom/realmode/x86_asm.S

@@ -150,7 +150,7 @@ __realmode_call:
 	mov	%ax, %es
 	mov	%ax, %fs
 	mov	%ax, %gs
-	mov	$0x40, %ax
+	mov	$RAM_DATA_ACPI_SEG, %ax
 	mov	%ax, %ds
 	pop	%ax