[libcxxabi] [mlir] [compiler-rt] [openmp] [libcxx] [clang] [lld] [llvm] [runtimes] Use LLVM libunwind from libc++abi by default (PR #77687)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

This looks like a simplified version of https://reviews.llvm.org/D150897 that does not remove redundant `LIBCXXABI_USE_LLVM_UNWINDER ON`. LGTM but I think some `docs/` changes are needed.

https://github.com/llvm/llvm-project/pull/77687