softfloat: Name rounding mode enum

softfloat: Name rounding mode enum

Give the previously unnamed enum a typedef name.  Use the packed
attribute so that we do not affect the layout of the float_status
struct.  Use it in the prototypes of relevant functions.

Adjust switch statements as necessary to avoid compiler warnings.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>