MINOR: threads: flatten the per-thread cpu-map

MINOR: threads: flatten the per-thread cpu-map

When we initially experimented with threads and processes support, we
needed to implement arrays of threads per process for cpu-map, but this
is not needed anymore since we support either threads or processes.
Let's simply make the thread-based cpu-map per thread and not per
thread and per process since that's not used anymore. Doing so reduces
the global struct from 33kB to 1.5kB.