Move MASK_ABS macro to libavcodec/mathops.h

This macro is only used in two places, both in libavcodec, so this
is a more sensible place for it.

Two small tweaks to the macro are made:

- removing the trailing semicolon
- dropping unnecessary 'volatile' from the x86 asm

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/mathops.h

@@ -138,6 +138,13 @@ if ((y) < (x)) {\
 }
 #endif
 
+#ifndef MASK_ABS
+#define MASK_ABS(mask, level) do {              \
+        mask  = level >> 31;                    \
+        level = (level ^ mask) - mask;          \
+    } while (0)
+#endif
+
 #ifndef NEG_SSR32
 #   define NEG_SSR32(a,s) ((( int32_t)(a))>>(32-(s)))
 #endif