Add a function to round a floatx80 to the defined precision
(floatx80_rounding_precision)

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <20170628204241.32106-5-laurent@vivier.eu>