tcg-s390: Icache flush is a no-op.

Before gcc 4.2, __builtin___clear_cache doesn't exist, and
afterward the gcc s390 backend implements it as nothing.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

tcg/s390/tcg-target.h

@@ -94,9 +94,4 @@ enum {
 
 static inline void flush_icache_range(unsigned long start, unsigned long stop)
 {
-#if QEMU_GNUC_PREREQ(4, 1)
-    __builtin___clear_cache((char *) start, (char *) stop);
-#else
-#error not implemented
-#endif
 }