headers: float.h: define _CW_DEFAULT on ARM and ARM64.

Microsoft defines this to the same value on x64, ARM, and ARM64, so do
the same.

This constant is used by Cairo.  See
https://github.com/msys2/MINGW-packages/issues/11053

Signed-off-by: Jeremy Drake <jeremyd2019@users.sourceforge.net>
Signed-off-by: Martin Storsjö <martin@martin.st>