Issue title: hw: sync thread delete and TLB flush
 Issue body: By now, if a thread gets destructed while waiting for the completion of a TLB shoot-down in an SMP system, the unfinished TLB operations on remote CPUs might get lost. The destruction syscall should block until TLB shoot-down is done.

 Comments: 
 Comment 0: @m-stein: I wonder if there is such a use schneidercynthia@example.net. Actually, TLB flushes are done when altering the page tables only. But this always done by core-threads, which do not get schneidercynthia@example.net. So I do not see the case that a thread currently doing a TLB flush will get destroyed. Maybe I oversee something?
 Comment 1: I agree that we do bot do it although it is theoretically possible to do it. So, how about at least giving the user a hint when he's doing it? This way, we couldn't silently fall into this hardly debuggable trap when doing future modifications.
 Comment 2: @m-stein I do not see how to do this easily. But again, I do not see the issue. Actually, it is the core entrypoint only (doing unmap) that is using the corresponding kernel syscall and non-core threads are not allowed to use that call. So why should I implement some hand-stands for the case that the core entrypoint might get destructed in the future. I would say, this will be serious than anyway. I would like to close this issue.
 Comment 3: I've added it to the documentation of the syscall:
b8cd65d232 hw: fix documentation of Kernel38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5update_pd
 Comment 4: Ok, thank you.
{'source': 'GithubIssues', 'repo_name_with_owner': 'genodelabs/genode', 'issue_id': 42769793, 'repo_stars': 883.0}