`times` should always return 0 for `tms_cutime` (#510)

`tms_cutime` is the sum of the user times of child processes *excluding
the current process*. Since WASI doesn't provide a way to spawn a new
process, this value should always be 0.

libc-bottom-half/clocks/times.c

@@ -17,7 +17,8 @@ clock_t times(struct tms *buffer) {
     __wasi_timestamp_t user = __clock();
     *buffer = (struct tms){
         .tms_utime = user,
-        .tms_cutime = user
+        // WASI doesn't provide a way to spawn a new process, so always 0.
+        .tms_cutime = 0
     };
 
     __wasi_timestamp_t realtime = 0;