std.debug.SelfInfo: thread safety

This has been a TODO for ages, but in the past it didn't really matter
because stack traces are typically printed to stderr for which a mutex
is held so in practice there was a mutex guarding usage of `SelfInfo`.

However, now that `SelfInfo` is also used for simply capturing traces,
thread safety is needed. Instead of just a single mutex, though, there
are a couple of different mutexes involved; this helps make critical
sections smaller, particularly when unwinding the stack as `unwindFrame`
doesn't typically need to hold any lock at all.

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