Clear X87 register state before using double.

MMX and X87 floating point instructions cannot be mixed freely on
the 32 bit x86 architecture.

This fixes a lot of unit tests in the 32bit build with
--enable-ext-intra.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1196

Change-Id: I0e1c3565f4b9cb4fc2d716e94d9c40e68b36fac8
6 files changed