[kernel] Add "noreturn" syscall option

The new "noreturn" option tag on syscall methods causes those methods to
be generated with [[noreturn]] / _Noreturn to avoid clang complaining
that other functions marked noreturn, like exit(), because it can't tell
that the syscall never returns.

src/libraries/j6/j6/syscalls.h.cog

@@ -39,7 +39,11 @@ for id, scope, method in syscalls.methods:
         for type, suffix in param.type.c_names(param.options):
             args.append(f"{type} {param.name}{suffix}")
 
-    cog.outl(f"""j6_status_t j6_{name} ({", ".join(args)});""")
+    attr = ""
+    if "noreturn" in method.options:
+        attr += "_Noreturn "
+
+    cog.outl(f"""{attr}j6_status_t j6_{name} ({", ".join(args)});""")
 ]]]*/
 /// [[[end]]]