thread: stash domain id to work around vtophys problems on ppc64

Adding to zombie list can be perfomed by idle threads, which on ppc64 leads to
panics as it requires a sleepable lock.

Reported by: alfredo
Reviewed by: kib, markj
Fixes: r367842 ("thread: numa-aware zombie reaping")
Differential Revision: https://reviews.freebsd.org/D27288