[libunwind] [libunwind] [SEH] Implement parsing of aarch64 pdata/xdata (PR #137949)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> The parsing itself looks fine. I don't have any context for how this code is supposed to work, though.

Thanks! Yeah the whole context isn't entirely clear to me as well, but roughly I think the idea is that we normally run `RtlUnwindEx` for unwinding an exception, but this is a case where we'd be manually unwinding the stack and calling the personality function ourself, so we need to dig it up from the xdata. In any case - this does the aarch64 equivalent of the existing x86_64 code above.

https://github.com/llvm/llvm-project/pull/137949