x86util: import MOVHL macro

Originally committed to x264 in 1637239a by Henrik Gramner who has
agreed to re-license it as LGPL.  Original commit message follows.

    x86: Avoid some bypass delays and false dependencies

    A bypass delay of 1-3 clock cycles may occur on some CPUs when transitioning
    between int and float domains, so try to avoid that if possible.
