Add implementation of MMX __m64 functions for MSVC x64.

Add implementation of MMX __m64 functions for MSVC x64.

Microsoft C++ does not define __m64 and all related MMX functions in
x64.  However, it succeeds in generating object files for SSE2 code
inside pixman.

The real problem happens during linking, when it cannot find MMX functions
(which are not defined as intrinsics for AMD64 platform).

I have implemented those missing functions using general programming.

MMX __m64 is used relatively scarcely within SSE2 implementation, and the
performance impact probably is negligible.

Bug 22390.