samples/mpu/mpu_stack_guard_test: Whitelist x86_64
samples/mpu/mpu_stack_guard_test: Whitelist x86_64

No MPU support there yet.  This test should really be predicated on a
kconfig variable, not architecture.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr