cmake: Move syscall_macros.h generation into the build stage

This fixes https://github.com/zephyrproject-rtos/zephyr/issues/5186



The script that generates syscall_macros.h is moved from Configuration
time to build time. This allows us to express to the build system that
syscall_macros.h depends on the script that generates it.

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