tcg: Reload local variables after return from longjmp

Recent compilers look deep into cpu_exec, find longjmp as a noreturn
function and decide to smash some stack variables as they won't be used
again. This may lead to env becoming invalid after return from setjmp,
causing crashes. Fix it by reloading env from cpu_single_env in that
case.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

cpu-exec.c

@@ -587,6 +587,10 @@ int cpu_exec(CPUState *env)
                 /* reset soft MMU for next block (it can currently
                    only be set by a memory fault) */
             } /* for(;;) */
+        } else {
+            /* Reload env after longjmp - the compiler may have smashed all
+             * local variables as longjmp is marked 'noreturn'. */
+            env = cpu_single_env;
         }
     } /* for(;;) */