nb/intel/gm45: Enable 64bit support

This patch does the following:
- Allow selecting 64bit from Kconfig
- Fix up integer to pointer conversion that gcc complains about
- Add a buildtest target in configs

Tested on Thinkpad X200: boots fine to the payload

Change-Id: Icb9c31a28ee231b87109b19c00ce2f8b48b5aefe
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64095
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/configs/config.lenovo_t400_vboot_and_debug b/configs/config.lenovo_t400_vboot_and_debug
index 2923f94..39c17e6 100644
--- a/configs/config.lenovo_t400_vboot_and_debug
+++ b/configs/config.lenovo_t400_vboot_and_debug
@@ -12,3 +12,4 @@
 CONFIG_DEBUG_ADA_CODE=y
 CONFIG_H8_FN_KEY_AS_VBOOT_RECOVERY_SW=y
 CONFIG_VBOOT=y
+CONFIG_USE_EXP_X86_64_SUPPORT=y