std.Io.Threaded: set poll_buffer_len to 32

reasoning is that polling with large amount of operations will be rarely
done with std.Io.Threaded. However this still provides the opportunity
to provide concurrency for any real world use cases that need it.

lib/std/Io/Threaded.zig

@@ -1323,7 +1323,7 @@ fn waitForApcOrAlert() void {
 
 const max_iovecs_len = 8;
 const splat_buffer_size = 64;
-const poll_buffer_len = 100;
+const poll_buffer_len = 32;
 const default_PATH = "/usr/local/bin:/bin/:/usr/bin";
 
 comptime {