[ulib][launchpad] Calculate space for handle array in initial stack size

The initial stack size calculation is meant to cover what the process
will need to read the bootstrap message.  That needs not just the message
buffer but also the handle array.

Bug: ZX-2926 #comment launchpad includes handle array size
Test: CQ
Change-Id: I1608b1d64ecf9ddcd8b69fa14c9d26fae52b8a51