Auto merge of #127397 - jyn514:multi-thread-panic-hook, r=workingjubilee

fix interleaved output in the default panic hook when multiple threads panic simultaneously

previously, we only held a lock for printing the backtrace itself. since all threads were printing to the same file descriptor, that meant random output in the default panic hook from one thread would be interleaved with the backtrace from another. now, we hold the lock for the full duration of the hook, and the output is ordered.

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i noticed some odd things while working on this you may or may not already be aware of.

- libbacktrace is included as a submodule instead of a normal rustc crate, and as a result uses `cfg(backtrace_in_std)` instead of a more normal `cfg(feature = "rustc-dep-of-std")`. probably this is left over from before rust used a cargo-based build system?
- the default panic handler uses `trace_unsynchronized`, etc, in `sys::backtrace::print`. as a result, the lock only applies to concurrent *panic handlers*, not concurrent *threads*.  in other words, if another, non-panicking, thread tried to print a backtrace at the same time as the panic handler, we may have UB, especially on windows.
    - we have the option of changing backtrace to enable locking when `backtrace_in_std` is set so we can reuse their lock instead of trying to add our own.

Author: bors