accel/tcg/user-exec: silence the compiler warnings

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
../accel/tcg/user-exec.c: In function ‘handle_cpu_signal’:
../accel/tcg/user-exec.c:169:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
  169 |             cpu_exit_tb_from_sighandler(cpu, old_set);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../accel/tcg/user-exec.c:172:9: note: here
  172 |         default:

Mark the cpu_exit_tb_from_sighandler() function with QEMU_NORETURN to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20201211152426.350966-8-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

accel/tcg/user-exec.c

@@ -49,7 +49,8 @@ __thread uintptr_t helper_retaddr;
 /* exit the current TB from a signal handler. The host registers are
    restored in a state compatible with the CPU emulator
  */
-static void cpu_exit_tb_from_sighandler(CPUState *cpu, sigset_t *old_set)
+static void QEMU_NORETURN cpu_exit_tb_from_sighandler(CPUState *cpu,
+                                                      sigset_t *old_set)
 {
     /* XXX: use siglongjmp ? */
     sigprocmask(SIG_SETMASK, old_set, NULL);