Define mm_flags/support to be 0 on architectures where they don't exist.

Make sure we don't try to set them if that's the case.
This lets dct-test and fft-test build with --disable-mmx on x86, but not yet on ARM.

Originally committed as revision 13957 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/dsputil.h

@@ -578,6 +578,11 @@ extern int mm_flags;
 #define DECLARE_ALIGNED_8(t, v) DECLARE_ALIGNED(16, t, v)
 #define STRIDE_ALIGN 16
 
+#else
+
+#define mm_flags 0
+#define mm_support() 0
+
 #endif
 
 #ifndef DECLARE_ALIGNED_8