64 bit version made similar to 32 bit with registers 1 for load and store results, and 2 and 3 as expanded float temporary values.

TEST=out/Release/libyuv_unittest --gtest_filter=*Half*

BUG=libyuv:560
R=wangcheng@google.com

Review URL: https://codereview.chromium.org/2467723002 .