[sim,rx] Silence warning that turns into a build error

On a 32-bit build, I ran into the following:

sim/rx/fpu.c:789:6: error: "*((void *)&a+8)" may be used uninitialized in this function [-Werror=maybe-uninitialized]
rv = fp_implode (&a);

To silence this, just initialize the struct with 0's.

sim/rx/ChangeLog:

2021-04-09  Luis Machado  <luis.machado@linaro.org>

	* fpu.c (rxfp_itof): Initialize structure.

sim/rx/fpu.c

@@ -732,7 +732,7 @@ rxfp_itof (long fa, int round_mode)
   int sign = 0;
   unsigned int frac_bits;
   volatile unsigned int whole_bits;
-  FP_Parts a;
+  FP_Parts a = {0};
 
   if (fa == 0)
     return PLUS_ZERO;