cmake: Re-organize syscall generation wrt. the build system

This commit fixes
https://github.com/zephyrproject-rtos/zephyr/issues/5008

.

It does so by splitting up gen_syscalls.py into two scripts with a
json metadata file to communicate syscall metadata between them. The
parsing script parses header files from include/ and writes syscall
metadata to a file if the contents changed. The generation script
reads from the json file and generates syscall code.

The build system DAG now looks like this:

always_rebuild -> json -> syscalls -> offset.o

The script for generating json will do so only if the content changes,
this ensures that the entire DAG does not always do a full rebuild.

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