[libcxx] [libunwind] [DRAFT][libc++] Switch FreeBSD to C++26 (PR #86658)

[Ed Maste via cfe-commits]

emaste wrote:

> The C++26 job is part of the first jobs that get run'
Oops, I did miss that. I think this would be the only buildkite job using `generic-cxx26`

https://github.com/llvm/llvm-project/pull/86658