cpu/x86/64bit: Add a separate Makefile.inc

Follow-up patches will add more to this makefile.

Change-Id: I8da6265b4c810e39a67f5ec27e26eeb26e3679a4
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63758
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
diff --git a/src/cpu/x86/64bit/Makefile.inc b/src/cpu/x86/64bit/Makefile.inc
new file mode 100644
index 0000000..721e620
--- /dev/null
+++ b/src/cpu/x86/64bit/Makefile.inc
@@ -0,0 +1,7 @@
+bootblock-y += mode_switch.S
+ifneq ($(CONFIG_VBOOT_STARTS_BEFORE_BOOTBLOCK),y)
+verstage-y += mode_switch.S
+endif
+romstage-y += mode_switch.S
+postcar-y += mode_switch.S
+ramstage-y += mode_switch.S