x86inc: Be more verbose in assertion failures

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/x86/x86inc.asm

@@ -295,7 +295,7 @@ DECLARE_REG_TMP_SIZE 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14
 
 %macro ASSERT 1
     %if (%1) == 0
-        %error assert failed
+        %error assertion ``%1'' failed
     %endif
 %endmacro