boards: qemu_x86: enable icount mode
boards: qemu_x86: enable icount mode

Enable icount mode for qemu_x86 platform, The icount shift value is
selectd based on cpu clock frequency of this platform. The virtual
cpu will execute one instruction every 2^shift ns of virtual time.

Signed-off-by: Wentong Wu <a92ffab6ab5ff780cf36418e13a69a97399df0f5@intel.com>

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