PPC: check for x-form asm constraint support

Originally committed as revision 18606 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/ppc/intreadwrite.h

@@ -24,6 +24,8 @@
 #include <stdint.h>
 #include "config.h"
 
+#if HAVE_XFORM_ASM
+
 #define AV_RL16 AV_RL16
 static inline uint16_t AV_RL16(const void *p)
 {
@@ -93,6 +95,8 @@ static inline void AV_WL64(void *p, uint64_t v)
 
 #endif /* HAVE_LDBRX */
 
+#endif /* HAVE_XFORM_ASM */
+
 /*
  * GCC fails miserably on the packed struct version which is used by
  * default, so we override it here.