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>

kernel/CMakeLists.txt

@@ -23,7 +23,16 @@ add_library(kernel
   version.c
   work_q.c
   smp.c
-)
+  )
+
+# Kernel files has the macro __ZEPHYR_SUPERVISOR__ set so that it
+# optimizes the code when userspace is enabled.
+set_target_properties(
+  kernel
+  PROPERTIES
+  COMPILE_DEFINITIONS
+  __ZEPHYR_SUPERVISOR__
+  )
 
 target_sources_ifdef(CONFIG_INT_LATENCY_BENCHMARK kernel PRIVATE int_latency_bench.c)
 target_sources_ifdef(CONFIG_STACK_CANARIES        kernel PRIVATE compiler_stack_protect.c)
@@ -43,6 +52,7 @@ target_sources_ifdef(
   userspace.c
   )
 
+
 add_dependencies(kernel offsets_h)
 
 target_link_libraries(kernel zephyr_interface)