avutil/cpu: force mmx on selection of higher x86 SIMD features

Fixes various runtime failures with manually set flags that represent no
existing CPU

Fixes Ticket3653
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/cpu.c

@@ -47,6 +47,26 @@
 static int flags, checked;
 
 void av_force_cpu_flags(int arg){
+    if (   (arg & ( AV_CPU_FLAG_3DNOW    |
+                    AV_CPU_FLAG_3DNOWEXT |
+                    AV_CPU_FLAG_SSE      |
+                    AV_CPU_FLAG_SSE2     |
+                    AV_CPU_FLAG_SSE2SLOW |
+                    AV_CPU_FLAG_SSE3     |
+                    AV_CPU_FLAG_SSE3SLOW |
+                    AV_CPU_FLAG_SSSE3    |
+                    AV_CPU_FLAG_SSE4     |
+                    AV_CPU_FLAG_SSE42    |
+                    AV_CPU_FLAG_AVX      |
+                    AV_CPU_FLAG_XOP      |
+                    AV_CPU_FLAG_FMA3     |
+                    AV_CPU_FLAG_FMA4     |
+                    AV_CPU_FLAG_AVX2     ))
+        && !(arg & AV_CPU_FLAG_MMX)) {
+        av_log(NULL, AV_LOG_WARNING, "MMX implied by specified flags\n");
+        arg |= AV_CPU_FLAG_MMX;
+    }
+
     flags   = arg;
     checked = arg != -1;
 }