fpu/softfloat: re-factor add/sub

We can now add float16_add/sub and use the common decompose and
canonicalize functions to have a single implementation for
float16/32/64 add and sub functions.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

include/fpu/softfloat.h

@@ -236,6 +236,10 @@ float64 float16_to_float64(float16 a, flag ieee, float_status *status);
 /*----------------------------------------------------------------------------
 | Software half-precision operations.
 *----------------------------------------------------------------------------*/
+
+float16 float16_add(float16, float16, float_status *status);
+float16 float16_sub(float16, float16, float_status *status);
+
 int float16_is_quiet_nan(float16, float_status *status);
 int float16_is_signaling_nan(float16, float_status *status);
 float16 float16_maybe_silence_nan(float16, float_status *status);