cpu-exec: avoid repeated sigsetjmp on interrupts

The sigsetjmp only needs to be prepared once for the whole execution
of cpu_exec.  This patch takes care of the "== 0" side, using a
nested loop so that cpu_handle_interrupt goes straight back to
cpu_handle_exception without doing another sigsetjmp.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpu-exec.c

@@ -627,14 +627,11 @@ int cpu_exec(CPUState *cpu)
     for(;;) {
         /* prepare setjmp context for exception handling */
         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
+            /* if an exception is pending, we execute it here */
+            while (!cpu_handle_exception(cpu, &ret)) {
                 TranslationBlock *last_tb = NULL;
                 int tb_exit = 0;
 
-            /* if an exception is pending, we execute it here */
-            if (cpu_handle_exception(cpu, &ret)) {
-                break;
-            }
-
                 while (!cpu_handle_interrupt(cpu, &last_tb)) {
                     TranslationBlock *tb = tb_find(cpu, last_tb, tb_exit);
                     cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
@@ -642,6 +639,9 @@ int cpu_exec(CPUState *cpu)
                        if the guest is in advance */
                     align_clocks(&sc, cpu);
                 }
+            }
+            break;
+
         } else {
 #if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
             /* Some compilers wrongly smash all local variables after