x86: arch.h: remove reverse dependency
x86: arch.h: remove reverse dependency

kernel.h depends on arch.h, and reverse dependencies need to be
removed. Define k_tid_t as some opaque pointer type so that arch.h
doesn't have to pull in kernel.h.

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

Vudentz/zephyr,nashif/zephyr,mbolivar/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,aceofall/zephyr-iotos,kraj/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,explora26/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,ldts/zephyr,zephyriot/zephyr,nashif/zephyr,zephyriot/zephyr,finikorg/zephyr,punitvara/zephyr,nashif/zephyr,mbolivar/zephyr,finikorg/zephyr,mbolivar/zephyr,zephyriot/zephyr,punitvara/zephyr,galak/zephyr,punitvara/zephyr,zephyriot/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,nashif/zephyr,kraj/zephyr