semihosting: Bound length for semihost_sys_{read,write}

semihosting: Bound length for semihost_sys_{read,write}

Fixes a minor bug in which a 64-bit guest on a 32-bit host could
truncate the length.  This would only ever cause a problem if
there were no bits set in the low 32, so that it truncates to 0.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>