std: pass the Thread for the newly spawned thread to the platform

[joboet]

This PR moves the responsibility for calling set_current (which initialises the thread handle) to the platform implementation. This unblocks #144465 and cleans up the fallout from #144500, as well as enabling the removal of the set_name function on platforms without thread names (as we now have access to the thread name in the platform thread main function).

This PR also contains a second commit (sorry, I couldn't help myself) that fixes a (mostly theoretical) unsoundness introduced by #115746, which added a call to current_os_id to the SIGSEGV signal handler even though that function might not be async-signal-safe. The thread ID is now initialised before the stack overflow information, so the ID can be stored therein.

[rustbot]

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[orlp]

This actually conflicts with #144465 due to parallel work. @joboet Can you take a look?

[bors]

☔ The latest upstream changes (presumably #147090) made this pull request unmergeable. Please resolve the merge conflicts.