[sgen] Use regular, non-safe LLS foreach macro.

[sgen] Use regular, non-safe LLS foreach macro.

As it turns out, all these uses of FOREACH_THREAD_SAFE were inside regions
where the suspend lock is held. This effectively meant that no modifications
to the thread list could actually happen while the foreach was executing. Since
the guarantees provided by FOREACH_THREAD_SAFE are not needed in any of these
cases, switch to FOREACH_THREAD. The latter macro also happens to be much more
lightweight, so this should result in an overall (small) performance gain.