kernel: rename z_arch_ to arch_

Promote the private z_arch_* namespace, which specifies
the interface between the core kernel and the
architecture code, to a new top-level namespace named
arch_*.

This allows our documentation generation to create
online documentation for this set of interfaces,
and this set of interfaces is worth treating in a
more formal way anyway.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/scripts/gen_syscalls.py b/scripts/gen_syscalls.py
index ef410db..d70befb 100755
--- a/scripts/gen_syscalls.py
+++ b/scripts/gen_syscalls.py
@@ -189,7 +189,7 @@
         mrsh_args[5:] = ["(uintptr_t) &more"]
 
     syscall_id = "K_SYSCALL_" + func_name.upper()
-    invoke = ("z_arch_syscall_invoke%d(%s)"
+    invoke = ("arch_syscall_invoke%d(%s)"
               % (len(mrsh_args),
                  ", ".join(mrsh_args + [syscall_id])))