avutil/x86/pixelutils: Empty MMX state in ff_pixelutils_sad_8x8_mmxext

We currently mostly do not empty the MMX state in our MMX
DSP functions; instead we only do so before code that might
be using x87 code. This is a violation of the System V i386 ABI
(and maybe of other ABIs, too):
"The CPU shall be in x87 mode upon entry to a function. Therefore,
every function that uses the MMX registers is required to issue an
emms or femms instruction after using MMX registers, before returning
or calling another function." (See 2.2.1 in [1])
This patch does not intend to change all these functions to abide
by the ABI; it only does so for ff_pixelutils_sad_8x8_mmxext, as this
function can by called by external users, because it is exported
via the pixelutils API. Without this, the following fragment will
assert (on x86/x64):
    uint8_t src1[8 * 8], src2[8 * 8];
    av_pixelutils_sad_fn fn = av_pixelutils_get_sad_fn(3, 3, 0, NULL);
    fn(src1, 8, src2, 8);
    av_assert0_fpu();

[1]: https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/intel386-psABI-1.1.pdf

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

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