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

[Michał Górny via cfe-commits]

mgorny wrote:

FTR, this breaks standalone builds against installed LLVM's libunwind since `unwind_shared` target doesn't exist. However, as long as I can pass `-DLIBCXXABI_USE_LLVM_UNWINDER=OFF` that just does the right thing, it's fine with me.

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