Increase slab-block size in signal component

The backend allocator for the slab is a sliced heap, which hands out
allocations with page-size granularity (4096 bytes). Therefore, the
slab-block size should also be about a multiple of the page size minus
some bytes of overhead.

Additional adjustments:

- The slab-block size and the default quota-upgrade amount for SIGNAL
  sessions depends on the platform bit width now.
- The signal test also stresses the case of many managed context in one
  session including creation and destruction of the used signal receiver
  in repeated rounds.