target/ppc: Zero second doubleword for VSX madd instructions

In 205eb5a we updated most VSX instructions to zero the
second doubleword, as is requested by PowerISA since v3.1.
However, VSX_MADD helper was left behind unchanged, while it
is also affected and should be fixed as well.

This patch applies the fix for MADD instructions.

Fixes: 205eb5a ("target/ppc: Change VSX instructions behavior to fill with zeros")
Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220906125523.38765-6-victor.colombo@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

target/ppc/fpu_helper.c

@@ -2167,7 +2167,7 @@ VSX_TSQRT(xvtsqrtsp, 4, float32, VsrW(i), -126, 23)
 void helper_##op(CPUPPCState *env, ppc_vsr_t *xt,                             \
                  ppc_vsr_t *s1, ppc_vsr_t *s2, ppc_vsr_t *s3)                 \
 {                                                                             \
-    ppc_vsr_t t = *xt;                                                        \
+    ppc_vsr_t t = { };                                                        \
     int i;                                                                    \
                                                                               \
     helper_reset_fpstatus(env);                                               \