MINOR: clock: use ltid_bit in clock_report_idle()

MINOR: clock: use ltid_bit in clock_report_idle()

Since commit cc7a11ee3 ("MINOR: threads: set the tid, ltid and their bit
in thread_cfg") we ought not use (1UL << thr) to get the group mask for
thread <thr>, but (ha_thread_info[thr].ltid_bit). clock_report_idle()
needs this.

This also implies not using all_threads_mask anymore but taking the mask
from the tgroup since it becomes relative now.