cpu-exec: Fix compiler warning (-Werror=clobbered)

Reloading of local variables after sigsetjmp is only needed for some
buggy compilers.

The code which should reload these variables causes compiler warnings
with gcc 4.7 when compiler optimizations are enabled:

cpu-exec.c:204:15: error:
variable ‘cpu’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
cpu-exec.c:207:15: error:
variable ‘cc’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
cpu-exec.c:202:28: error:
argument ‘env’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]

Now this code is only used for compilers which need it
(and gcc 4.5.x, x > 0 which does not need it but won't give warnings).

There were bug reports for clang and gcc 4.5.0, while gcc 4.5.1
was reported to work fine without the reload code. For clang it
is not clear which versions are affected, so simply keep the status quo
for all clang compilations. This can be improved later.

Backports commit 0448f5f8b816923b198ab6c32286fd1f3b2f3e45 from qemu

qemu/cpu-exec.c

@@ -238,14 +238,27 @@ int cpu_exec(struct uc_struct *uc, CPUState *cpu)
                    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'. */
+#if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
+            /* Some compilers wrongly smash all local variables after
+             * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
+             * Reload essential local variables here for those compilers.
+             * Newer versions of gcc would complain about this code (-Wclobbered). */
             cpu = uc->current_cpu;
             env = cpu->env_ptr;
             cc = CPU_GET_CLASS(uc, cpu);
 #ifdef TARGET_I386
             x86_cpu = X86_CPU(uc, cpu);
 #endif
+#else /* buggy compiler */
+            /* Assert that the compiler does not smash local variables. */
+            g_assert(cpu == current_cpu);
+            g_assert(cc == CPU_GET_CLASS(cpu));
+#ifdef TARGET_I386
+            g_assert(x86_cpu == X86_CPU(cpu));
+            g_assert(env == &x86_cpu->env);
+#endif
+#endif /* buggy compiler */
+            cpu->can_do_io = 1;
         }
     } /* for(;;) */