syscall: Move arch specific syscall code into its own header

Split out the arch specific syscall code to reduce include pollution
from other arch related headers.  For example on ARM its possible to get
errno.h included via SoC specific headers.  Which created an interesting
compile issue because of the order of syscall & errno/errno syscall
inclusion.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

include/syscall.h

@@ -12,7 +12,7 @@
 #include <zephyr/types.h>
 #include <syscall_list.h>
 #include <syscall_macros.h>
-#include <arch/cpu.h>
+#include <arch/syscall.h>
 
 #ifdef __cplusplus
 extern "C" {