Also use the _mm_broadcastsi128_si256 intrisic for
Apple clang versions 4.[012]

https://bugzilla.mozilla.org/show_bug.cgi?id=1085607
https://code.google.com/p/webm/issues/detail?id=1082

Change-Id: I6bc821d8163387194ef663e94bfed91fa7281d88