softfloat: Correctly handle NaNs in float16_to_float32()

Correctly handle NaNs in float16_to_float32(), by defining and
using a float16ToCommonNaN() function, as we do with the other formats.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 80d8cc4..3abd170 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -2761,9 +2761,7 @@
 
     if (aExp == 0x1f && ieee) {
         if (aSig) {
-            /* Make sure correct exceptions are raised.  */
-            float32ToCommonNaN(a STATUS_VAR);
-            aSig |= 0x200;
+            return commonNaNToFloat32(float16ToCommonNaN(a STATUS_VAR) STATUS_VAR);
         }
         return packFloat32(aSign, 0xff, aSig << 13);
     }