[libcxxabi][CMake] Set --unwindlib=none when using LLVM libunwind

We already link libunwind explicitly so avoid trying to link toolchain's
default libunwind which may be missing. This matches what we already do
for libcxx.

Differential Revision: https://reviews.llvm.org/D129469

NOKEYCHECK=True
GitOrigin-RevId: 1f3f4833eae7ec276530ad0c0018d46a01563420
1 file changed