softfloat: Fix the incorrect computation in float32_exp2

The float32_exp2 function is computing wrong exponent of 2.

For example, with the following set of values {0.1, 2.0, 2.0, -1.0},
the expected output would be {1.071773, 4.000000, 4.000000, 0.500000}.
Instead, the function is computing {1.119102, 3.382044, 3.382044, -0.191022}

Looking at the code, the float32_exp2() attempts to do this

                  2     3     4     5           n
  x        x     x     x     x     x           x
 e  = 1 + --- + --- + --- + --- + --- + ... + --- + ...
           1!    2!    3!    4!    5!          n!

But because of the typo it ends up doing

  x        x     x     x     x     x           x
 e  = 1 + --- + --- + --- + --- + --- + ... + --- + ...
           1!    2!    3!    4!    5!          n!

This is because instead of the xnp which holds the numerator, parts_muladd
is using the xp which is just 'x'.  Commit '572c4d862ff2' refactored this
function, and mistakenly used xp instead of xnp.

Cc: qemu-stable@nongnu.org
Fixes: 572c4d8 "softfloat: Convert float32_exp2 to FloatParts"
Partially-Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1623
Reported-By: Luca Barbato (https://gitlab.com/lu-zero)
Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Message-Id: <168304110865.537992.13059030916325018670.stgit@localhost.localdomain>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 1098cc3)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

fpu/softfloat.c

@@ -5135,7 +5135,7 @@ float32 float32_exp2(float32 a, float_status *status)
     float64_unpack_canonical(&rp, float64_one, status);
     for (i = 0 ; i < 15 ; i++) {
         float64_unpack_canonical(&tp, float32_exp2_coefficients[i], status);
-        rp = *parts_muladd(&tp, &xp, &rp, 0, status);
+        rp = *parts_muladd(&tp, &xnp, &rp, 0, status);
         xnp = *parts_mul(&xnp, &xp, status);
     }