target-s390: Use noreturn for exception and load_psw

Both always exit the cpu loop.
Signed-off-by: NRichard Henderson <rth@twiddle.net>

target-s390x/helper.h

 #include "exec/def-helper.h"
 
-DEF_HELPER_2(exception, void, env, i32)
+DEF_HELPER_2(exception, noreturn, env, i32)
 DEF_HELPER_FLAGS_4(nc, TCG_CALL_NO_WG, i32, env, i32, i64, i64)
 DEF_HELPER_FLAGS_4(oc, TCG_CALL_NO_WG, i32, env, i32, i64, i64)
 DEF_HELPER_FLAGS_4(xc, TCG_CALL_NO_WG, i32, env, i32, i64, i64)
@@ -90,7 +90,7 @@ DEF_HELPER_FLAGS_1(popcnt, TCG_CALL_NO_RWG_SE, i64, i64)
 #ifndef CONFIG_USER_ONLY
 DEF_HELPER_3(servc, i32, env, i64, i64)
 DEF_HELPER_4(diag, i64, env, i32, i64, i64)
-DEF_HELPER_3(load_psw, void, env, i64, i64)
+DEF_HELPER_3(load_psw, noreturn, env, i64, i64)
 DEF_HELPER_FLAGS_2(spx, TCG_CALL_NO_RWG, void, env, i64)
 DEF_HELPER_FLAGS_1(stck, TCG_CALL_NO_RWG_SE, i64, env)
 DEF_HELPER_FLAGS_2(sckc, TCG_CALL_NO_RWG, void, env, i64)