target/ppc: Set float_tininess_before_rounding at cpu reset

As defined in Power 3.0 section 4.4.4 "Underflow Exception",
a tiny result is detected before rounding.

Fixes: https://bugs.launchpad.net/qemu/+bug/1841491
Reported-by: Paul Clarke <pc@us.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190827020013.27154-1-richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target/ppc/translate_init.inc.c

@@ -10461,6 +10461,10 @@ static void ppc_cpu_reset(CPUState *s)
     s->exception_index = POWERPC_EXCP_NONE;
     env->error_code = 0;
 
+    /* tininess for underflow is detected before rounding */
+    set_float_detect_tininess(float_tininess_before_rounding,
+                              &env->fp_status);
+
     for (i = 0; i < ARRAY_SIZE(env->spr_cb); i++) {
         ppc_spr_t *spr = &env->spr_cb[i];