We get HOST_WORDS_BIGENDIAN from config-host.h, but the include
is missing. Fix it.

This fixes `make check-softfloat' on big endian hosts.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>