kernel: Cmake: Add __ZEPHYR_SUPERVISOR__ macro for kernel files.

Normally a syscall would check the current privilege level and then
decide to go to _impl_<syscall> directly or go through a
_handler_<syscall>.
__ZEPHYR_SUPERVISOR__ is a compiler optimization flag which will
make all the system calls from the kernel files directly link
to the _impl_<syscall>. Thereby reducing the overhead of checking the
privileges.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>