i386: Disable check_consistency for GCC 5 and above [BZ #25788]

check_consistency should be disabled for GCC 5 and above since there is
no fixed PIC register in GCC 5 and above.  Check __GNUC_PREREQ (5,0)
instead OPTIMIZE_FOR_GCC_5 since OPTIMIZE_FOR_GCC_5 is false with
-fno-omit-frame-pointer.

sysdeps/unix/sysv/linux/i386/sysdep.h

@@ -580,8 +580,9 @@ struct libc_do_syscall_args
 # define EXTRAVAR_5
 #endif
 
-/* Consistency check for position-independent code.  */
-#if defined __PIC__ && !defined OPTIMIZE_FOR_GCC_5
+/* Consistency check for position-independent code.  Disabled for GCC 5
+   and above since there is no fixed PIC register in GCC 5 and above.  */
+#if defined __PIC__ && !__GNUC_PREREQ (5,0)
 # define check_consistency()						      \
   ({ int __res;								      \
      __asm__ __volatile__						      \