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

[Vitaly Buka via cfe-commits]

vitalybuka wrote:

@petrhosek @ldionne 
would it be better detect libunwind in LLVM_ENABLE_RUNTIMES?

We have similar thing for lld in compiler-rt.
There are trade offs:
1. checking LLVM_ENABLE_RUNTIMES simplify out of the end user experience,
1. forcing default  ON simplifies cmake files and avoids unexpected use of system lib 




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