Revert "remove fldcw/fstcw from Win64 builds"

Revert "remove fldcw/fstcw from Win64 builds"

This reverts commit 60a3cb9ad840377d46286bfd703c30a4a4ee56e2.

Reason for revert: x87 instruction usage might not be as
clear cut as I would like. At the very least, llvm mingw
builds appear to having issues with emms.

Original change's description:
> remove fldcw/fstcw from Win64 builds
>
> _MCW_PC (Precision control) is not supported on x64:
> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/control87-controlfp-control87-2
>
> The x87 FPU is not used on Win64 or ARM so setting the x87 control word
> is not necessary. The SSE/SSE2 and ARM FPUs don't have a precision
> control - the precision is embedded in each instruction - so the need to
> set the control word is also gone.

BUG=webm:1500

Change-Id: I25bcfa96bc9c860f6c7e03315d75fa6fd1d88ec5