cmake: Execute arch/CMakeLists.txt before subsys/CMakeList.txt
cmake: Execute arch/CMakeLists.txt before subsys/CMakeList.txt

Re-order the execution of the arch/ and subsys/ CMakeLists.txt code to
work around a manifestation of issue #6505. When OpenThread created an
External project in subsys, it did not have access to important
toolchain flags added in arch/.

Intuitively, subsystems might depends on how the ARCH is configured,
but the ARCH shouldn't depend on any subsystem.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

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