cpu/x86/smm/smm_stub: Fix GDT for x86_64

The previous code was crashing when jumping back to ramstage, now it
works. The GDT is now using the same values as the other ones in
coreboot.

Change-Id: Id00467d9d8a4138ddea73adbda4b39f12def583f
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48214
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/cpu/x86/smm/smm_stub.S b/src/cpu/x86/smm/smm_stub.S
index 11ea9a7..56e2504 100644
--- a/src/cpu/x86/smm/smm_stub.S
+++ b/src/cpu/x86/smm/smm_stub.S
@@ -91,7 +91,7 @@
 
 	/* gdt selector 0x18, flat code segment (64-bit) */
 	.word	0xffff, 0x0000
-	.byte	0x00, 0x9b, 0xcf, 0x00
+	.byte	0x00, 0x9b, 0xaf, 0x00
 
 	/* gdt selector 0x20 tss segment */
 	.word   0xffff, 0x0000