WIP: Use LLVM libunwind on FreeBSD

[ararslan]

This switches nongnu libunwind for LLVM libunwind on FreeBSD. Doesn't fully work yet (profiling, notably).

[DilumAluthge]

Any chance that this is the same bug as the one being discussed in the following threads?

1. https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256864
2. https://bugs.llvm.org/show_bug.cgi?id=50972

[ararslan]

This now works entirely except for profiling, where the stack frames show up as unknown:

Overhead _ [+additional indent] Count File:Line; Function
=========================================================
   _101 [unknown stackframe]
   _ 101 [unknown stackframe]
   _  101 [unknown stackframe]
   _   101 [unknown stackframe]
100_    101 [unknown stackframe]
Total snapshots: 101 (100% utilization across all threads and tasks. Use the `groupby` kwarg to break down by thread and/or task)

I'm pretty far out of my depth here. If anybody is willing to help me figure this out, I would be very grateful. Otherwise I think we should mark the specific failing tests as broken on FreeBSD and merge this, as that situation is a strict improvement over what's on master currently.

[IanButterworth]

The [unknown stackframe] was a bug that is now fixed in #42226, but that means that there will now be no collected samples at all, so the issue remains

[DilumAluthge]

@ararslan Can this PR be closed now?

[ararslan]

It might be nice to do this eventually, since it seems convenient to me for macOS, FreeBSD, and eventually Linux to use the same unwinding library. But yeah, this PR is definitely no longer a priority for me so feel free to close if you'd like.

[vtjnash]

xref #30154