These inline implementations of AV_COPY64, AV_SWAP64 and AV_ZERO64
are known to clobber the FPU state - which has to be restored
with the 'emms' instruction afterwards.

This was known and signaled with the FF_COPY_SWAP_ZERO_USES_MMX
define, which calling code seems to have been supposed to check,
in order to call emms_c() after using them. See
0b1972d4,
29c4c088 and
df215e57 for history on earlier
fixes in the same area.

However, new code can use these AV_*64() macros without knowing
about the need to call emms_c().

Just get rid of these dangerous inline assembly snippets; this
doesn't make any difference for 64 bit architectures anyway.
Signed-off-by: Martin Storsjö <martin@martin.st>