softfloat: Extend float_exception_flags to 16 bits

We will shortly have more than 8 bits of exceptions.
Repack the existing flags into low bits and reformat to hex.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211119160502.17432-2-richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>