MINOR: thread: split out thread_bind_to_cpu_map()

MINOR: thread: split out thread_bind_to_cpu_map()

set_thread_cpu_affinity() binds the current thread to the CPUs that
the cpu_map assigns to it. The next commit will need to bind to
another thread's CPUs, around the first touch of an allocation, so
split a thread_bind_to_cpu_map(tgrp, ltid) helper out of it, binding
the current thread to the CPUs assigned to local thread <ltid> of
thread group <tgrp> (0-based). set_thread_cpu_affinity() keeps its
master-process check and delegates to the new function.