target-arm: Correct ABD's handling of negative zeroes

Implement ABD by taking the absolute value of the difference
of the operands (as the ARM ARM specifies) rather than by
flipping the order of the operands to the subtract based
on the results of a comparison. The latter approch gives
the wrong answers for some edge cases like negative zero.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/neon_helper.c

@@ -1789,9 +1789,7 @@ uint32_t HELPER(neon_abd_f32)(uint32_t a, uint32_t b)
 {
     float32 f0 = make_float32(a);
     float32 f1 = make_float32(b);
-    return float32_val((float32_compare_quiet(f0, f1, NFS) == 1)
-                    ? float32_sub(f0, f1, NFS)
-                    : float32_sub(f1, f0, NFS));
+    return float32_val(float32_abs(float32_sub(f0, f1, NFS)));
 }
 
 uint32_t HELPER(neon_add_f32)(uint32_t a, uint32_t b)