Export getThreadId
Sometimes one needs to convert ThreadId to Int (e.g in hashable). GHC.Conc.Sync is obscure enough place to expose this implementation detail.
Quick search shows that many do it wrongly (e.g. hashable), as the rts C definition has changed https://hackage-search.serokell.io/?q=rts_getThreadId
#22705 is related. (base itself could use CApiFFI for importing ThreadId# primitives).