postprocess.c currently has C, MMX, MMXEXT, 3DNow as well as
SSE2 versions of its internal functions. But given that only
ancient 32-bit x86 CPUs don't support SSE2, the MMX, MMXEXT
and 3DNow versions are obsolete and are therefore removed by
this commit. This saves about 56KB here.

(The SSE2 version in particular is not really complete,
so that it often falls back to MMXEXT (which means that
there were some identical (apart from the name) MMXEXT
and SSE2 functions; this duplication no longer exists
with this commit.)
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>