kernel: add config for Spectre V1 mitigation
kernel: add config for Spectre V1 mitigation

This is off by default, but may be selected by the arch
configuration.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

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