Mingw-w64, which is the source of this code, uses different
definitions of the rounding bits FE_TONEAREST and friends.
They immediately reflect the bit values in the FPU control word,
while on Cygwin they are shifted down to become the values 0-3.
Fix the bit computing expression to account for the difference.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>