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>