remove fldcw/fstcw from Win64 builds

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: I014513282a7dc320d1cdeaec48249d98a66bf09f